Swimmer runs to interrupt argument (5)
I believe the answer is:
sprat
'swimmer' is the definition.
(sprat is a kind of fish)
'runs to interrupt argument' is the wordplay.
'runs' becomes 'r' (cricket abbreviation).
'to interrupt' is an insertion indicator.
'argument' becomes 'spat' (I've seen this before).
'r' placed into 'spat' is 'SPRAT'.
(Other definitions for sprat that I've seen before include "Parts (anag.)" , "one on a fat-free diet" , "Traps (anag.)" , "Small sea fish" , "'Set a . . . . . to catch a herring'" .)