Only reason, you say, for being in the red (5)
I believe the answer is:
alone
'only' is the definition.
(synonyms)
'reason you say for being in the red' is the wordplay.
I cannot really understand how this works, but
'you' could be 'one' (I've seen this before) and 'one' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for alone that I've seen before include "On one's own" , "Exclusively; solitary" , "With no one to see" , "Solo" , "Single" .)