Getting old silver in front of gold (5)
I believe the answer is:
aging
'getting old' is the definition.
(I've seen this before)
'silver in front of gold' is the wordplay.
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'front of' suggests taking the first letters (the letter at the front of the word).
The first letter of 'gold' is 'g'.
'ag'+'in'+'g'='AGING'
(Other definitions for aging that I've seen before include "Getting older" , "Getting old" , "Becoming more grown-up" , "in decline" , "development" .)