Silver has new sign for nickname (7)
I believe the answer is:
agnomen
'for nickname' is the definition.
(I've seen this before)
'silver has new sign' is the wordplay.
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'has' means one lot of letters go next to another.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'sign' becomes 'omen' (omen is a kind of sign).
'ag'+'n'+'omen'='AGNOMEN'
(Other definitions for agnomen that I've seen before include "Roman extra name" , "Nickname" .)