Sign of life after sappers drive back (7)
I believe the answer is:
repulse
'drive back' is the definition.
(synonyms)
'sign of life after sappers' is the wordplay.
'sign of life' becomes 'pulse' (a person's pulse).
'after' says to put letters next to each other.
'sappers' becomes 'RE' (nickname for the Royal Engineers).
'pulse' after 're' is 'REPULSE'.
(Other definitions for repulse that I've seen before include "Cause to feel intense aversion" , "Drive back by force" , "Battlecruiser" , "Fend off" , "Disgust" .)