Mature women recruited by band (4,2)
I believe the answer is:
grow up
'mature' is the definition.
(growing up is a kind of maturing)
'women recruited by band' is the wordplay.
'women' becomes 'w' (standard abbreviation).
'recruited by' is an insertion indicator.
'band' becomes 'group' (I've seen this before).
'w' inserted within 'group' is 'GROW UP'.
(Other definitions for grow up that I've seen before include "Become an adult" , "Be your age, acquire maturity" , "Informal injunction to have sense, be mature" , "Become mature - or injunction to act like it?" , "Become mature, or an injunction to have sense" .)