Maturing alcohol in silver (5)
I believe the answer is:
aging
'maturing' is the definition.
(I've seen this before)
'alcohol in silver' is the wordplay.
'alcohol' becomes 'gin' (gin is a kind of alcohol).
'in' means one lot of letters goes inside another.
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'gin' going inside 'ag' is 'AGING'.
(Other definitions for aging that I've seen before include "Becoming more grown-up" , "Growing old in the US" , "in decline" , "Growing older" , "Getting old" .)