Yes. In Romanian, adjectives usually come after the noun, but you can place them before it, for a variety of reasons (some are the same as the reasons for switching the noun-adjective order in English). If before the noun, you apply the definite article to the adjective.
So this could be "Eu mănânc dejunul mic", but "micul dejun" is idiomatic for "breakfast".
Well.. you could say ”Eu mănânc mic-dejunul” (note the hyphen) in some poetry, but usually it is as potestasity said. Nobody says „Eu mănânc dejunul mic”, this expression „micul dejun” means ”the breakfast”, the first meal of the day, and the words' order is not ”reversible”. If you say "I eat some breakfast", or "a breakfast" you could use "Eu mănânc un mic dejun" and get rid of the dilemma... :P