Menace that involves sappers (6)
I believe the answer is:
threat
'menace' is the definition.
(I know this)
'that involves sappers' is the wordplay.
'involves' means one lot of letters goes inside another.
'sappers' becomes 'RE' (nickname for the Royal Engineers).
'that' going around 're' is 'THREAT'.
(Other definitions for threat that I've seen before include "Endangerment" , "Hatter (anag)" , "Risk" , "Statement of intention to harm" , "Possibility of danger" .)