Weaken fast, held down by soldiers (6)
I believe the answer is:
relent
'weaken' is the definition.
(relent can mean to weaken or decrease in intensity)
'fast held down by soldiers' is the wordplay.
'fast' becomes 'lent' (Lent is a period of fasting in Christianity).
'held down by' means one lot of letters go next to another (in a down clue, some letters go on top of others).
'soldiers' becomes 're' (Royal Engineers).
'lent' put after 're' is 'RELENT'.
(Other definitions for relent that I've seen before include "'Give in, soften (6)'" , "Be more lenient, abandon harsh intentions" , "Change one's mind" , "Abandon cruel treatment" , "Acquiesce" .)