Encourage knight - go back for a drink (6)
I believe the answer is:
eggnog
'a drink' is the definition.
(eggnog is a kind of drink)
'encourage knight go back' is the wordplay.
'encourage' becomes 'egg'.
'knight' becomes 'n' (chess).
'back' is a reversal indicator.
'go' in reverse letter order is 'og'.
'egg'+'n'+'og'='EGGNOG'
'for' is the link.
(Other definitions for eggnog that I've seen before include "Christmas drink" , "Rum-based drink" , "A punch for bedtime, perhaps" , "Drink (milk punch)" , "Christmas tipple" .)