Danger that must include sappers (6)
I believe the answer is:
threat
'danger' is the definition.
(threat is a kind of danger)
'that must include sappers' is the wordplay.
'must include' is an insertion indicator.
'sappers' becomes 're' (Royal Engineers).
'that' going around 're' is 'THREAT'.
(Other definitions for threat that I've seen before include "Potential source of danger" , "Menacing words" , "Hazard" , "Possibility of danger" , "Aggressive declaration" .)