Sign of life in bather swimming before lunch? (9)
I believe the answer is:
heartbeat
'sign of life' is the definition.
'bather swimming before lunch?' is the wordplay.
'swimming' indicates an anagram.
'before' is a charade indicator (letters next to each other).
'lunch?' becomes 'eat' (to lunch can mean to eat lunch).
'bather' anagrammed gives 'heartb'.
'heartb'+'eat'='HEARTBEAT'
'in' is the link.
(Other definitions for heartbeat that I've seen before include "Circulation noise" , "Animating force" , "Pulse of cardiac origin" , "Essential pulsation" .)