Stand up to disheartened reservist (6)
I believe the answer is:
resist
'stand up to' is the definition.
(I've seen this before)
'disheartened reservist' is the wordplay.
'disheartened' means to remove the middle letters.
'reservist' with its centre removed is 'RESIST'.
(Other definitions for resist that I've seen before include "Put up opposition to sister" , "Oppose or fight against someone" , "Withstand or defy" , "Refuse to comply" , "Take stand against, oppose" .)