Danger which catches engineers (6)
I believe the answer is:
threat
'danger' is the definition.
(threat is a kind of danger)
'which catches engineers' is the wordplay.
'which' becomes 'that'.
'catches' means one lot of letters goes inside another.
'engineers' becomes 're' (Royal Engineers).
'that' placed around 're' is 'THREAT'.
(Other definitions for threat that I've seen before include "A warning of harm to come" , "Impending danger" , "Hazard" , "Warning of intent to harm" , "Hatter (anag)" .)