Money, when backing, to put on (5)
I believe the answer is:
notes
'money' is the definition.
(I've seen this before)
'backing to put on' is the wordplay.
'backing' is a reversal indicator.
'to put' becomes 'set' (both can mean to place or position).
'set'+'on'='seton'
'seton' backwards is 'NOTES'.
'when' is the link.
(Other definitions for notes that I've seen before include "Short missives" , "Short written messages" , "output from musicians" , "Memoranda" , "Brief letters; money" .)