Refute objection by engineers (5)
I believe the answer is:
rebut
'refute' is the definition.
(I've seen this before)
'objection by engineers' is the wordplay.
'objection' becomes 'but'.
'by' says to put letters next to each other.
'engineers' becomes 're' (Royal Engineers).
'but' after 're' is 'REBUT'.
(Other definitions for rebut that I've seen before include "Check" , "Rebuff; deny" , "Claim or prove to be false" , "Prove to be false or incorrect" , "Refure or disprove" .)