Monster blunder from the defence troops (4)
I believe the answer is:
ogre
'monster' is the definition.
(fairy tale monster)
'blunder from the defence troops' is the wordplay.
'blunder from the defence' becomes 'og' (I can't explain this - if you can you should give a lot more credence to this answer).
'troops' becomes 'RE' (Royal Engineers army regiment).
'og'+'re'='OGRE'
Can you help me to learn more?
(Other definitions for ogre that I've seen before include "Jack's giant perhaps" , "terrible person" , "Ergo (anag.)" , "Cruel giant of folklore" , "One to gobble flesh" .)