Only one (4)
I believe the answer is:
sole
'only one' is the definition.
This is all the clue.
(Other definitions for sole that I've seen before include "O __ Mio (Neapolitan song)" , "Bottom of the foot" , "fish > that's never less than foot in length" , "Kind of fish - lose it" , "A lone sort of fish - lose it" .)