Many a key part to of no credit (5)
I believe the answer is:
debit
'credit' is the definition.
(thesaurus)
'many a key part' is the wordplay.
'many' becomes 'd'.
'a key' becomes 'e' (musical key).
'part' becomes 'bit part' (both mean a piece of something).
'd'+'e'+'bit'='DEBIT'
'to of no' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for debit that I've seen before include "It's not to one's credit" , "Opposite of credit" , "Record of a sum owed" , "Sum owing (in accounts)" , "Charge" .)