Employ engineer, mature (6)
I believe the answer is:
engage
'employ' is the definition.
(I know that employ can be written as engage)
'engineer mature' is the wordplay.
'engineer' becomes 'eng' (abbreviation).
'mature' becomes 'age' (synonyms).
'eng'+'age'='ENGAGE'
(Other definitions for engage that I've seen before include "Become bound; attract" , "Hold" , "Employ or take on" , "Join" , "Employ or pledge" .)