Repeated phrase over in American state — definitely (6)
I believe the answer is:
affirm
'state definitely' is the definition.
'affirm' can be an answer for 'state' (I've seen this before). I am unsure of the 'definitely' bit.
'repeated phrase over in american' is the wordplay.
'repeated phrase' becomes 'riff' (I've seen this in another clue).
'over' says the letters should be written backwards.
'in' indicates putting letters inside.
'american' becomes 'Am' (abbreviation in some contexts).
'riff' back-to-front is 'ffir'.
'ffir' going into 'am' is 'AFFIRM'.
Can you help me to learn more?
(Other definitions for affirm that I've seen before include "Verify" , "Ratify; vouch" , "Endorse" , "Declare, assert" , "Confidently put" .)