Red troops and girl living together (9)
I believe the answer is:
commensal
'living together' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'red troops and girl' is the wordplay.
'red troops' becomes 'commen' (I can't explain this - if you can you should give a lot more credence to this answer).
'and' is a charade indicator (letters next to each other).
'girl' becomes 'Sal' (feminine given name).
'commen'+'sal'='COMMENSAL'
Can you help me to learn more?
(Another definition for commensal that I've seen is " Eating at the same table".)