Potential killer of fish in strait and river (9)
I believe the answer is:
strangler
'potential killer' is the definition.
'strangler' can be an answer for 'killer' (strangler is a kind of killer). I'm not sure about the 'potential' bit.
'fish in strait and river' is the wordplay.
I cannot really see how this works, but
'fish' could be 'angler' (angler fish is a kind of fish) and 'angler' is located in the answer.
'strait' could be 'st' (I've seen this in other clues) and 'st' is located in the answer.
'river' could be 'r' (abbreviation for river) and 'r' is present in the answer.
This accounts for all the letters.
This may be the basis of clue (or it may be nonsense).
'of' acts as a link.
Can you help me to learn more?
(Other definitions for strangler that I've seen before include "Killer compressing the windpipe" , "he could take your breath away" , "Murderer" , "Killer by choking" , "criminal" .)