Discovered secret: party not right to spend money needlessly (7)
I believe the answer is:
overpay
'spend money needlessly' is the definition.
I can't tell whether this definition defines the answer.
'discovered secret party not right' is the wordplay.
'discovered secret' becomes 'over' (I can't justify this - if you can you should give a lot more credence to this answer).
'not' suggests deleting specific letters.
'right' becomes 'rt' (abbreviation as in Rt Hon- the Right Honourable).
'party' with 'rt' removed is 'pay'.
'over'+'pay'='OVERPAY'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for overpay that I've seen before include "Give more than enough" , "Give too much money" .)