Number needing a drink, not small! (6)
I believe the answer is:
thirty
'number' is the definition.
(thirty is a kind of whole number)
'needing a drink not small' is the wordplay.
'needing a drink' becomes 'thirsty' (I've seen this before).
'not' is a deletion indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'thirsty' with 's' taken out is 'THIRTY'.
(Other definitions for thirty that I've seen before include "Roman number XXX" , "Score and a half" , "Half of threescore" , "Six times five" , "The number of silver coins for betrayal" .)