Surrender after Royal Engineers retreat (6)
I believe the answer is:
recede
'retreat' is the definition.
(I know that retreat can be written as recede)
'surrender after royal engineers' is the wordplay.
'surrender' becomes 'cede' (synonyms).
'after' says to put letters next to each other.
'royal engineers' becomes 'RE' (abbreviation).
'cede' after 're' is 'RECEDE'.
(Other definitions for recede that I've seen before include "Decree (anag) - ebb" , "Become more distant" , "Decline" , "Start balding" , "Go backwards" .)