Last of liquid: tip it down sink (5)
I believe the answer is:
'sink' is the definition.
'last of liquid? tip it down' is the wordplay.
'last of' says to take the final letters.
'tip it down' becomes 'rain' (UK phrase meaning to rain).
The final letter of 'liquid' is 'd'.
(Other definitions for drain that I've seen before include "Waste channel" , "Remove liquid from" , "Siphon off" , "Bleed" , "Exhaust - pipe" .)