Old in the head, ultimately? (5)
I believe the answer is:
adult
'old' is the definition.
(thesaurus)
'in the head ultimately?' is the wordplay.
'in' says the answer is hidden in the clue.
'ADULT' can be found hidden inside 'head ultimately'.
(Other definitions for adult that I've seen before include "one no longer in minority" , "I wasn't born yesterday" , "Mature, fully grown" , "X-rated?" , "Mature - grown-up" .)