Mature person in lead ultimately (5)
I believe the answer is:
adult
'mature person' is the definition.
(I've seen this before)
'in lead ultimately' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ADULT' is hidden within 'lead ultimately'.
(Other definitions for adult that I've seen before include "Mature - grown-up" , "Supposedly "responsible" person" , "not fit for children to see" , "One no longer young" , "used to describe dirty movies" .)