Discredit military engineers only (5)
I believe the answer is:
rebut
'discredit' is the definition.
(I've seen this in another clue)
'military engineers only' is the wordplay.
'military engineers' becomes 'RE' (Royal Engineers in the British Army).
'only' becomes 'but' (but can mean only - eg 'life is but a dream').
're'+'but'='REBUT'
(Other definitions for rebut that I've seen before include "Give the lie to" , "explode" , "Scotch" , "Prove the claim wrong" , "Refute - disprove" .)