The ad modified, being very unpopular (5)
I believe the answer is:
hated
'very unpopular' is the definition.
(someone hated is very unpopular)
'the ad modified' is the wordplay.
'modified' is an anagram indicator.
'the'+'ad'='thead'
'thead' anagrammed gives 'HATED'.
'being' acts as a link.
(Other definitions for hated that I've seen before include "Despised" , "Abhorred" , "Unpopular" , "was unable to stand" , "couldn't stand" .)