Fish for only one (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)
'only one' is the second definition.
'for' acts as a link.
(Other definitions for sole that I've seen before include "A fish; part of shoe" , "A lone flat fish?" , "Flatfish; exclusive" , "Unique" , "Lone; part of foot" .)