Cheers old boy overturning love ban ... (5)
I believe the answer is:
taboo
'ban' is the definition.
(I've seen this in another clue)
'cheers old boy overturning love' is the wordplay.
'cheers' becomes 'ta' (both can informally mean thank you).
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'overturning' says the letters should be written backwards.
'love' becomes 'o' (love means zero in tennis).
'ob' written backwards gives 'bo'.
'ta'+'bo'+'o'='TABOO'
(Other definitions for taboo that I've seen before include "Not permitted" , "Inhibition based on social custom" , "Prohibited by religion or custom" , "Beyond the pale" , "Anathema" .)