Reason to speak about money being returned (6)
I believe the answer is:
sanity
'reason' is the definition.
(I've seen this before)
'to speak about money being returned' is the wordplay.
'to speak' becomes 'say' (I've seen this in other clues).
'about' is an insertion indicator.
'money' becomes 'tin' (old-fashioned slang term for money).
'being returned' says the letters should be written backwards.
'tin' written backwards gives 'nit'.
'say' enclosing 'nit' is 'SANITY'.
(Other definitions for sanity that I've seen before include "Sound sense" , "Mental health" , "Reason and coherence" , "Soundness of mind" , "Mental wellness" .)