Enterprising bachelor getting on (4)
I believe the answer is:
bold
'enterprising' is the definition.
(similar in meaning)
'bachelor getting on' is the wordplay.
'bachelor' becomes 'b' (abbreviation e.g. BA=Bachelor of Arts).
'getting on' becomes 'old' ('getting on a bit' means 'old').
'b'+'old'='BOLD'
(Other definitions for bold that I've seen before include "Audacious" , "Confidently courageous" , "Thick and heavy (type)" , "Prominent" , "Daring, maybe impudent" .)