Engineers fast to slacken (6)
I believe the answer is:
relent
'slacken' is the definition.
(thesaurus)
'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 "Soften - yield" , "Ease off" , "Back down" , "Decide to be kind after all?" , "Give in to influence or pressure" .)