Engineers fast to yield (6)
I believe the answer is:
relent
'yield' is the definition.
('relent' can be a synonym of 'yield')
'engineers fast' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'fast' becomes 'lent' (I've seen this before).
're'+'lent'='RELENT'
'to' acts as a link.
(Other definitions for relent that I've seen before include "Be more lenient, abandon harsh intentions" , "Back down" , "'Give in, soften (6)'" , "Decide to be kind after all?" , "Go easy" .)