He’s storing change from a petty amount of money (10)
I believe the answer is:
shoestring
'he's storing change from a petty amount of money' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'he's storing change' is the wordplay.
'change' indicates anagramming the letters.
'hes'+'storing'='hesstoring'
'hesstoring' with letters rearranged gives 'SHOESTRING'.
Can you help me to learn more?
(Other definitions for shoestring that I've seen before include "Barely sufficient funds" , "Lace" , "for securing a trainer?" , "A minimum of money" , "Minimal amount of money" .)