While yes, in the phrase "having a rest," "rest" is a noun and "having" is the verb, the translative value would be "having a rest." This would be considered a verbal phrase, with "a rest" being used adverbially to modify "having." What is he doing? He's "having a rest." He is not "having," he's "having a rest." . The phrase as a whole, rather than the individual words, is being used to define the word. However, I still don't like it, because "having a rest" is not idiomatic English with which I'm familiar. "Taking a nap," perhaps, which conveys the same idea I suppose, so one can't really be angry about it.. But maybe this phrase is used more often elsewhere?