Deny I had flipped in agreement (10)
I believe the answer is:
contradict
'deny' is the definition.
(I've seen this before)
'i had flipped in agreement' is the wordplay.
'i had' becomes 'id' (I'd).
'flipped' says the letters should be written in reverse.
'in' indicates putting letters inside.
'agreement' becomes 'contract' (contract is a kind of agreement**).
'id' in reverse letter order is 'di'.
'di' placed within 'contract' is 'CONTRADICT'.
(Other definitions for contradict that I've seen before include "Argue against" , "Assert the opposite of (a statement)" , "Refute" , "dispute" , "Be in conflict with" .)