Fish unaccompanied (4)
I believe the answer is:
sole
I believe this is a double definition.
'fish' is the first definition.
(sole is a kind of fish)
'unaccompanied' is the second definition.
(Other definitions for sole that I've seen before include "Kind of fish - lose it" , "swimmer" , "Lone; part of foot" , "piece of leather" , "One and only; part of foot" .)