Check at the back for definite sign of life (7)
I believe the answer is:
repulse
'check' is the definition.
(I've seen this before)
'at the back for definite sign of life' is the wordplay.
'at the back' suggests the final letters.
'sign of life' becomes 'pulse' (I've seen this before).
The final letters of 'for definite' are 're'.
're'+'pulse'='REPULSE'
(Other definitions for repulse that I've seen before include "kick in the teeth" , "Drive back by force" , "Beat back" , "Repel" , "Disgust" .)