Only part of a foot (4)
I believe the answer is:
sole
I believe this is a double definition.
'only' is the first definition.
(I know that only can be written as sole)
'part of a foot' is the second definition.
(I know that sole is part of foot)
(Other definitions for sole that I've seen before include "piece of leather" , "Underside of footwear" , "Fish - alone" , "A lone flat fish?" , "Dover - -" .)