About to interrupt swimmer’s short rest (8)
I believe the answer is:
breather
'short rest' is the definition.
(I've seen this before)
'about to interrupt swimmer's' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'to interrupt' indicates putting letters inside.
'swimmer' becomes 'bather' (synonyms).
're' put within 'bather' is 'BREATHER'.
(Other definitions for breather that I've seen before include "One's probably not dying" , "Short rest between bouts of exercise" , "lung, possibly?" , "Short pause for rest" , "Recovery time" .)