One holding the purse strings in power may, somehow, by changing rates (9)
I believe the answer is:
paymaster
'one holding the purse strings' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'power may somehow by changing rates' is the wordplay.
'power' becomes 'p'.
'somehow' indicates anagramming the letters.
'by' is a charade indicator (letters next to each other).
'changing' is an anagram indicator.
'may' anagrammed gives 'aym'.
'rates' with letters rearranged gives 'aster'.
'p'+'aym'+'aster'='PAYMASTER'
'in' is the link.
Can you help me to learn more?
(Other definitions for paymaster that I've seen before include "Official dealing with wages" , "Official handing out money to troops or workers" , "funding source" , "Government official" , "One controlling through money" .)