Head of silver, if not grey (7)
I believe the answer is:
sunless
'grey' is the definition.
(I've seen this before)
'head of silver if not' is the wordplay.
'head of' says to take the initial letters.
'if not' becomes 'unless' (I've seen this before).
The first letter of 'silver' is 's'.
's'+'unless'='SUNLESS'
(Other definitions for sunless that I've seen before include "dim, certainly not bright" , "Offering nothing for a tanner" , "dark" , "Unlit by solar light" , "Clouded over" .)