Run in churlish swimmer (7)
I believe the answer is:
oarfish
'run in churlish swimmer' is the definition.
'oarfish' can be an answer for 'swimmer' (oarfish is a kind of aquatic vertebrate). I am unsure of the 'run in churlish' bit.
'run in churlish swimmer' is the wordplay.
'run' becomes 'r' (cricket abbreviation).
'in' indicates putting letters inside.
'churlish' becomes 'oa' (I can't justify this - if you can you should give a lot more credence to this answer).
'swimmer' becomes 'fish' (fish swim).
'oa'+'fish'='oafish'
'r' placed within 'oafish' is 'OARFISH'.
Can you help me to learn more?
(Other definitions for oarfish that I've seen before include "Big swimmer" , "Ocean swimmer" .)