Money in a sack is growing less (7)
I believe the answer is:
abating
'growing less' is the definition.
The answer and definition can be both related to change as well as being present participle verbs.
Perhaps you can see an association between them that I don't see?
'money in a sack' is the wordplay.
'money' becomes 'tin' (old-fashioned slang term for money).
'in' means one lot of letters goes inside another.
'sack' becomes 'bag' (I've seen this in another clue).
'a'+'bag'='abag'
'tin' put within 'abag' is 'ABATING'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for abating that I've seen before include "Dying away, slacking off" , "Getting less" , "Subsiding" , "Becoming less intense" , "Slacking off, dyeing away" .)