Engineers are unable to withdraw (6)
I believe the answer is:
recant
'withdraw' is the definition.
(I've seen this before)
'engineers are unable to' is the wordplay.
'engineers' becomes 're' (abbreviation for Royal Engineers).
'are unable to' becomes 'cant' (cannot).
're'+'cant'='RECANT'
(Other definitions for recant that I've seen before include "Renounce a former opinion or belief" , "Repudiate (belief)" , "Go back on a statement of belief" , "Disown" , "Deny" .)