Agreement backing troops in difficult situation (6)
I believe the answer is:
ordeal
'difficult situation' is the definition.
'agreement backing troops' is the wordplay.
'agreement' becomes 'deal' (deal is a kind of agreement**).
'backing' means one lot of letters go next to another.
'troops' becomes 'or' (military abbreviation for other ranks).
'deal' after 'or' is 'ORDEAL'.
'in' is the link.
(Other definitions for ordeal that I've seen before include "A tough time or hardship" , "Horrific experience" , "Prolonged distressing experience" , "severe test shows" , "A difficult and harrowing experience" .)