I had one to return to fool (5)
I believe the answer is:
idiot
'to fool' is the definition.
(synonyms)
'i had one to return' is the wordplay.
'i had' becomes 'i'd' (contraction).
'one' becomes 'i' (Roman numeral).
'return' is a reversal indicator.
'to' back-to-front is 'ot'.
'id'+'i'+'ot'='IDIOT'
(Other definitions for idiot that I've seen before include "Ignoramus's" , "Ass, nincompoop" , "Ass, ninny" , "Jackass" , "Foolish character" .)