Revolutionary shot sappers, the monster (4)
I believe the answer is:
ogre
'the monster' is the definition.
(fairy tale monster)
'revolutionary shot sappers' is the wordplay.
'revolutionary' is a reversal indicator.
'shot' becomes 'go' (both can mean an attempt).
'sappers' becomes 'RE' (nickname for the Royal Engineers).
'go' written backwards gives 'og'.
'og'+'re'='OGRE'
(Other definitions for ogre that I've seen before include "Giant who eats people" , "Ergo (anag.)" , "stern one who inspires fear" , "Has a monstrous role" , "Cruel giant of folklore" .)