Run into one who is trying to sell fish (5)
I believe the answer is:
trout
'fish' is the definition.
(trout is a kind of fish)
'run into one who' is the wordplay.
'run' becomes 'r' (cricket abbreviation).
'into' means one lot of letters goes inside another.
'one who' becomes 'tout' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'r' going inside 'tout' is 'TROUT'.
'is trying to sell' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for trout that I've seen before include "Tickled fish?" , "unpleasant old woman" , "It rises to fly" , "Small salmonlike fish" , "Unattractive old woman" .)