Determined, the missing husband can overturn promises (9)
I believe the answer is:
tenacious
'determined' is the definition.
(I've seen this before)
'the missing husband can overturn promises' is the wordplay.
'missing' indicates named letters should be taken away.
'husband' becomes 'h' (abbreviation).
'overturn' is a reversal indicator.
'promises' becomes 'IOUs' (promises to pay a debt).
'the' with 'h' removed is 'te'.
'can' in reverse letter order is 'nac'.
'te'+'nac'+'ious'='TENACIOUS'
(Other definitions for tenacious that I've seen before include "Holding fast" , "refusing to give up" , "Keeping a firm hold" , "Dogged" , "Holding or sticking firmly" .)