Engineers take forty days to ease off (6)
I believe the answer is:
relent
'ease off' is the definition.
(I've seen this before)
'engineers take forty days' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'take' means one lot of letters go next to another.
'forty days' becomes 'lent' (I've seen this before).
're'+'lent'='RELENT'
'to' is the link.
(Other definitions for relent that I've seen before include "Ease off" , "Become less intense" , "Capitulate" , "Become less severe, soften" , "Soften - yield" .)