Money returned regarding fertiliser ingredient (5)
I believe the answer is:
nitre
'fertiliser ingredient' is the definition.
(I've seen this in another clue)
'money returned regarding' is the wordplay.
'money' becomes 'tin' (old-fashioned slang term for money).
'returned' shows that the letters should be reversed in order.
'regarding' becomes 're' ('re' means regarding or about).
'tin' back-to-front is 'nit'.
'nit'+'re'='NITRE'
(Other definitions for nitre that I've seen before include "constituent of gunpowder" , "what goes into bangers" , "fertiliser" , "Also known as salt-petre" , "A part of fertilizer" .)