In party I'd find source of wealth (3,4)
I believe the answer is:
pay dirt
'source of wealth' is the definition.
(I've seen this in another clue)
'in party i'd' is the wordplay.
'party i'd' can be anagrammed to 'PAY DIRT'.
Though, I'm unsure how the anagram is indicated.
'find' acts as a link.
Can you help me to learn more?
(Other definitions for pay dirt that I've seen before include "profitable success" , "Rich ore-bearing ground (US)" , "Ground containing ore in profitable amounts" .)