"if it’s a definition you want, it’s a definition you’ll get"
"That said, definitions are useful. So, if it’s a definition you want, it’s a definition you’ll get. For the most part, physicists are trying to do the following:"
Does this sentence have any hidden agenda? Is it related to any quotes or proverbs?
The phrases containing "what you want" and "what you get" do exist in popular culture in many contexts like https://www.youtube.com/watch?v=fNq9_2bdeiM or or , and there are many more similar, but there is not any specific idiom like that, or I do not know of any - so its meaning is somewhat blurred to me.
However, in this case, a meaning that is closest to my intuition, is that the outcome that you get from your research may be distorted by what you wanted to get or what you expected at the time when you started to look for the result. So, the sentence "if it’s a definition you want, it’s a definition you’ll get" may mean that from the very beginning of scientific research, there is a threat, that you do not discover the truth / the right definition / but your result / your definition / is somewhat limited by your volition or your expectations.
This is a perfectly safe thing to say. It can have different subtexts but that is almost completely dependant on tone. As br0d4 said this is quite a common way to construct a phrase in English, you restate the request and plainly state that you will do that.
For example, my boss asks me for a full accounting of my time and I say that I'd prefer to give a rough outline. He insists so I reply, "ok, if you want a full accounting then you will get a full accounting" and I deliver him detailed records indicating that I should be billing him for more hours than I have been (and then I bill him). The subtext here is that I don't think we should be doing this but if the boss insists then I'll do it.
I offer tea to my guests but one of them hesitantly asks for something a bit more exotic (lemon maybe) and I say "If you want lemon then I'll get you lemon". The subtext here is I'm happy to fulfil my guests requests.
In your example I think the subtext is that the speaker doesn't think a definition is necessary but is willing to humour the requester and softens this slightly by stating that definitions are useful.