A number needing a drink out of the sun (6)
I believe the answer is:
thirty
'a number' is the definition.
(thirty is a kind of number)
'needing a drink out of the sun' is the wordplay.
'needing a drink' becomes 'thirsty' (I've seen this before).
'out of' is a deletion indicator.
'the sun' becomes 's'.
'thirsty' with 's' taken out is 'THIRTY'.
(Other definitions for thirty that I've seen before include "Six times five" , "November's days" , "Roman number XXX" , "The number of silver coins for betrayal" , "Sum of first four perfect squares" .)