Kitty and I agree to agree in essence (13)
I believe the answer is:
fundamentally
'in essence' is the definition.
(synonyms)
'kitty and i agree to agree' is the wordplay.
'kitty' becomes 'fund' (fund or money for a particular purpose).
'and' says to put letters next to each other.
'i agree' becomes 'amen' ('amen' might be said to indicate agreement).
'to agree' becomes 'tally' ('tally' can be a synonym of 'agree').
'fund'+'amen'+'tally'='FUNDAMENTALLY'
(Other definitions for fundamentally that I've seen before include "In essence" , "At heart" , "At bottom" .)