Yield to engineers fast (6)
I believe the answer is:
relent
'yield' is the definition.
(I know that yield can be written as relent)
'engineers fast' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'fast' becomes 'lent' (Lent is a period of fasting in Christianity).
're'+'lent'='RELENT'
'to' acts as a link.
(Other definitions for relent that I've seen before include "Give in to influence or pressure" , "Come round" , "Soften, grow less severe" , "Become less stern than intended" , "Be less strict" .)