A bit of a lad ultimately fully grown (5)
I believe the answer is:
adult
'fully grown' is the definition.
(synonyms)
'a bit of a lad ultimately' is the wordplay.
'a bit of' indicates a hidden word.
'ADULT' can be found hidden inside 'lad ultimately'.
(Other definitions for adult that I've seen before include "not green" , "Person of mature age" , "Supposedly "responsible" person" , "I wasn't born yesterday" , "used to describe dirty movies" .)