Maturing alcoholic spirit in silver (5)
I believe the answer is:
aging
'maturing' is the definition.
(I've seen this in another clue)
'alcoholic spirit in silver' is the wordplay.
'alcoholic spirit' becomes 'gin' (I've seen this before).
'in' indicates putting letters inside.
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'gin' inserted inside 'ag' is 'AGING'.
(Other definitions for aging that I've seen before include "development" , "Becoming more grown-up" , "Getting older" , "Getting old" , "Growing old in the US" .)