Great advertisement for an old man (7)
I believe the answer is:
grandad
'for an old man' is the definition.
(elderly relative)
'great advertisement' is the wordplay.
'great' becomes 'grand' (I've seen this before).
'advertisement' becomes 'ad' (short for advert).
'grand'+'ad'='GRANDAD'
(Other definitions for grandad that I've seen before include "And drag your ancestor here" , "relative" , "Dad rang his ancestor" , "senior citizen" , "Father's father" .)