Engineers interrupting swimmer's break (8)
I believe the answer is:
breather
'break' is the definition.
(I know that breather is a type of relief)
'engineers interrupting swimmer's' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'interrupting' is an insertion indicator.
'swimmer' becomes 'bather' (synonyms).
're' going inside 'bather' is 'BREATHER'.
(Other definitions for breather that I've seen before include "Short rest between bouts of exercise" , "Short pause for rest" , "One's probably not dying" , "One who inspires" , "Informally a pause for rest - or to take in air" .)