In difficult situation having finished active service? (2,1,4)
I believe the answer is:
up a tree
'in difficult situation' is the definition.
I don't know anything about this answer so I can't judge whether this works.
'finished active service?' is the wordplay.
I cannot really understand how this works, but
'finished' could be 'up' (eg 'time's up' means the time is finished) and 'up' is present in the answer.
'active' could be 'a' (grammatical abbreviation) and 'a' is found within the answer.
The remaining letters 'tree' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'having' acts as a link.
Can you help me to learn more?
(Other definitions for up a tree that I've seen before include "In slangy difficulties, maybe on branch" , "In great difficulties, like the rackaby baby" , "Like a lumberjack" , "American in need of assistance" .)