Not a creek, exactly, but there may be fish in it (5)
I believe the answer is:
creel
'it' is the definition.
The definition suggests a singular noun which matches the answer.
'not a creek exactly but there may be fish' is the wordplay.
I cannot really understand how this works, but
'fish' could be 'eel' (eel is a type of fish) and 'eel' is found in the answer.
This explanation may well be incorrect...
'in' is the link.
Can you help me to learn more?
(Other definitions for creel that I've seen before include "Bobbin rack" , "Wicker basket for fish" , "lobster basket" , "Container for fish" , "Angler's wicker basket" .)