Opening number's warble cut short (7)
I believe the answer is:
nostril
'opening' is the definition.
(nostril is a kind of opening)
'number's warble cut short' is the wordplay.
'number's' becomes 'nos' (abbreviation for 'number').
'warble' becomes 'trill' ('trill' can be a synonym of 'warble').
'cut short' means to remove the last letter.
'trill' with its final letter removed is 'tril'.
'nos'+'tril'='NOSTRIL'
(Other definitions for nostril that I've seen before include "Breathing opening for a human" , "A hole in the head" , "Part of nose" , "Bit of a breather" , "Breathing space" .)