Engineers left in charge for survival (5)
I believe the answer is:
relic
'for survival' is the definition.
(I have seen 'Ancient survival ' mean 'relic' so perhaps 'survival' could also mean 'relic')
'engineers left in charge' is the wordplay.
'engineers' becomes 're' (abbreviation for Royal Engineers).
'left' becomes 'l' (common abbreviation).
'in charge' becomes 'ic' (abbreviation for 'in charge').
're'+'l'+'ic'='RELIC'
(Other definitions for relic that I've seen before include "Piece left over" , "Part of a holy person's body or possession" , "Object from antiquity" , "Something surviving but outdated" , "Surviving ancient custom" .)