Facing a stag in trouble (7)
I believe the answer is:
against
'facing' is the definition.
(I've seen this before)
'a stag in trouble' is the wordplay.
'trouble' is an anagram indicator.
'stag'+'in'='stagin'
'stagin' anagrammed gives 'gainst'.
'a'+'gainst'='AGAINST'
(Other definitions for against that I've seen before include "Abutting" , "Opposed" , "Not in favour of" , "Placed in direct contact with" , "Opposite to" .)