Alien female's number (5)
I believe the answer is:
ether
'number' is the definition.
(I've seen this before)
'alien female's' is the wordplay.
'alien' becomes 'et' (extra terrestrial).
'female' becomes 'her' (I've seen this before).
'et'+'her'='ETHER'
(Other definitions for ether that I've seen before include "Gas - air - space" , "An early anaesthetic" , "Anaesthetic liquid; clear sky" , "It could knock you out" , "I'm volatile" .)