Forbidden love on flying boat (5)
I believe the answer is:
taboo
'forbidden' is the definition.
(synonyms)
'love on flying boat' is the wordplay.
'love' becomes 'o' (love means zero in tennis).
'on' indicates putting letters inside.
'flying' indicates an anagram (letters fly around into new positions).
'boat' anagrammed gives 'tabo'.
'o' going within 'tabo' is 'TABOO'.
(Other definitions for taboo that I've seen before include "Prohibited by religion or custom" , "Not allowed, forbidden, proscribed" , "(Irrational) prohibition" , "Ritually forbidden" , "Ritual prohibition" .)