Needing drink, son pops out to get three by ten? (6)
I believe the answer is:
thirty
'three by ten?' is the definition.
(30 is 3 x 10)
'needing drink son pops out' is the wordplay.
'needing drink' becomes 'thirsty' (someone thirsty is in need of a drink).
'son' becomes 's' (genealogical abbreviation for son).
'pops out' suggests deleting specific letters.
'thirsty' with 's' taken out is 'THIRTY'.
'to get' acts as a link.
(Other definitions for thirty that I've seen before include "Sum of first four perfect squares" , "Roman number XXX" , "November's days" , "Number of pieces of silver paid to Judas" , "Both RU teams" .)