Soldiers are unable to withdraw (6)
I believe the answer is:
recant
'withdraw' is the definition.
(thesaurus)
'soldiers are unable to' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'are unable to' becomes 'cant'.
're'+'cant'='RECANT'
(Other definitions for recant that I've seen before include "Revoke a former declaration" , "Formally reject a former belief" , "Withdraw former statement or belief" , "Renounce (a heretical belief)" , "Withdrawn former statement or belief" .)