Drunk, though a round behind (6)
I believe the answer is:
lapped
'drunk' is the definition.
(lapping is a kind of drinking)
'a round behind' is the wordplay.
'a round' becomes 'lap' (as in a round or lap of a park).
'behind' becomes 'ped' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'lap'+'ped'='LAPPED'
'though' acts as a link.
Can you help me to learn more?
(Other definitions for lapped that I've seen before include "Took up with the tongue" , "Made extra circuit" , "Washed or flowed against" , "Drank" , "Scooped up with the tongue" .)