Number needing a drink losing seconds (6)
I believe the answer is:
thirty
'number' is the definition.
(thirty is a kind of number)
'needing a drink losing seconds' is the wordplay.
'needing a drink' becomes 'thirsty' (I've seen this before).
'losing' suggests deleting specific letters.
'seconds' becomes 's' (abbreviation).
'thirsty' with 's' taken away is 'THIRTY'.
(Other definitions for thirty that I've seen before include "November's days" , "Both RU teams" , "Number of pieces of silver paid to Judas" , "Half of threescore" , "Roman number XXX" .)