You shouldn't have breast reduction! It's forbidden (5)
I believe the answer is:
taboo
'forbidden' is the definition.
(synonyms)
'you shouldn't have breast reduction' is the wordplay.
'you shouldn't have' becomes 'ta' (both can mean 'thank you').
'breast' becomes 'boob' (synonyms).
'reduction' means to remove the last letter.
'boob' with its last letter taken off is 'boo'.
'ta'+'boo'='TABOO'
'it's' is the link.
(Other definitions for taboo that I've seen before include "(Irrational) prohibition" , "By religion or custom, not to be done or touched" , "ruled out" , "Not to be touched or used, banned for religious reasons" , "Forbidden for social reasons" .)