Warning that restrains sappers (6)
I believe the answer is:
threat
'warning' is the definition.
(I know that threat is a type of warning)
'that restrains sappers' is the wordplay.
'restrains' is an insertion indicator.
'sappers' becomes 'RE' (nickname for the Royal Engineers).
'that' placed around 're' is 'THREAT'.
(Other definitions for threat that I've seen before include "An intimidating warning" , "Statement of harmful intent" , "Risk" , "Possibility of danger" , "Hatter (anag)" .)