Engineers put in that warning (6)
I believe the answer is:
threat
'warning' is the definition.
(threat is a kind of warning)
'engineers put in that' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'put in' is an insertion indicator.
're' placed within 'that' is 'THREAT'.
(Other definitions for threat that I've seen before include "Menace of impending evil" , "Endangerment" , "A warning of harm to come" , "Declaration of intention to harm another" , "An intimidatory warning" .)