Drink surely is potable, first of all (3)
I believe the answer is:
sip
'drink' is the definition.
(sipping is a kind of drinking)
'surely is potable first of all' is the wordplay.
'first of all' says to take the initial letters.
The initial letters of 'surely is potable' is 'SIP'.
(Other definitions for sip that I've seen before include "Small mouthful of liquid" , "Small mouthful of fluid" , "A small taste" , "Drink, small mouthful" , "Take in slowly" .)