One and only (4)
I believe the answer is:
sole
'one and only' is the definition.
(synonyms)
This is all the clue.
(Other definitions for sole that I've seen before include "A lone flat fish?" , "Underside of footwear" , "Fish - alone" , "A lone sort of fish - lose it" , "O __ Mio (Neapolitan song)" .)