Only part of a shoe (4)
I believe the answer is:
sole
I believe this is a double definition.
'only' is the first definition.
(synonyms)
'part of a shoe' is the second definition.
(I've seen this in another clue)
(Other definitions for sole that I've seen before include "Fish - alone" , "swimmer" , "O __ Mio (Neapolitan song)" , "One and only; part of foot" , "Unique" .)