Drink with the left back (7)
I believe the answer is:
support
'back' is the definition.
(I've seen this before)
'drink with the left' is the wordplay.
'drink' becomes 'sup' (I've seen this before**).
'with' says to put letters next to each other.
'the left' becomes 'port' (left side of a ship - opposite of starboard).
'sup'+'port'='SUPPORT'
(Other definitions for support that I've seen before include "Bolster or back up" , "Buttress" , "Prop" , "Shore up" , "Keep from falling, as crutch does (7)" .)