Repay engineers completely (7)
I believe the answer is:
requite
'repay' is the definition.
(I know that repay can be written as requite)
'engineers completely' is the wordplay.
'engineers' becomes 're' (abbreviation for Royal Engineers).
'completely' becomes 'quite' (synonyms in phrases like 'are you quite done?').
're'+'quite'='REQUITE'
(Other definitions for requite that I've seen before include "Make return" , "Quieter (anag.)" , "Appropriately, give back" .)