Secret supply of money, say (5)
I believe the answer is:
cache
'secret supply' is the definition.
'cache' can be an answer for 'supply' (thesaurus). I am not certain of the 'secret' bit.
'money say' is the wordplay.
'money' becomes 'cash' (I've seen this before).
'say' shows a homophone (sound like).
'cash' sounds like 'CACHE'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for cache that I've seen before include "Stockpile" , "Hiding-place for treasure etc" , "memory" , "Hidden storage space" , "Secret source" .)