Determined defender's disappointment (7)
I believe the answer is:
setback
'disappointment' is the definition.
(thesaurus)
'determined defender's' is the wordplay.
'determined' becomes 'set' ('set' can be a synonym of 'determined'**).
'defender' becomes 'back' (I've seen this in other clues).
'set'+'back'='SETBACK'
(Other definitions for setback that I've seen before include "unfortunate event" , "Hitch" , "Hindrance to progress" , "Problem" , "Snag" .)