Duke vacated seat taken by needy customer at bank (9)
I believe the answer is:
depositor
'customer at bank' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'duke vacated seat taken by needy' is the wordplay.
'vacated' means to remove the middle letters (internal letters vacate the word).
'seat' becomes 'sit' (synonyms).
'taken by' is an insertion indicator.
'needy' becomes 'poor' (similar in meaning).
'duke' with its middle removed is 'de'.
'sit' placed into 'poor' is 'positor'.
'de'+'positor'='DEPOSITOR'
Can you help me to learn more?
(Other definitions for depositor that I've seen before include "One putting money into a bank account" , "One handing over money" .)