Matures, holds on, settles (6)
I believe the answer is:
agrees
'settles' is the definition.
(settle can mean to agree or resolve)
'matures holds on' is the wordplay.
'matures' becomes 'ages' ('age' can be a synonym of 'mature').
'holds' indicates putting letters inside.
'on' becomes 're' ('on' can be mean 'with reference to').
'ages' going around 're' is 'AGREES'.
(Other definitions for agrees that I've seen before include "Accedes or concurs" , "Holds the same opinion" , "Assents, concurs" , "is in accord" , "Is of one mind" .)