A fine commercial for a relative (7)
I believe the answer is:
grandad
'for a relative' is the definition.
(grandad is a kind of relative)
'a fine commercial' is the wordplay.
'a fine' becomes 'grand' (I've seen this before).
'commercial' becomes 'ad' (abbreviation for advertisement).
'grand'+'ad'='GRANDAD'
(Other definitions for grandad that I've seen before include "senior citizen" , "And drag your ancestor here" , "relative" , "Dad rang his ancestor" , "Father's father" .)