Full responsibility is upon you and me (4)
I believe the answer is:
onus
'full responsibility' is the definition.
'onus' can be an answer for 'responsibility' (onus is a duty or responsibility). I am not certain of the 'full' bit.
'upon you and me' is the wordplay.
'upon' becomes 'on' (I've seen this in other clues).
'you and me' becomes 'us'.
'on'+'us'='ONUS'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for onus that I've seen before include "'Burden, of proof (4)'" , "Weight of responsibility" , "Encumbrance" , "Responsibilty" , "Liability" .)