Three Mobile number (5)
I believe the answer is:
ether
'number' is the definition.
(I've seen this before)
'three mobile' is the wordplay.
'mobile' is an anagram indicator.
'three' is an anagram of 'ETHER'.
(Other definitions for ether that I've seen before include "Gas - air - space" , "Old anaesthetic -- three (anag)" , "Solvent; clear sky (poet.)" , "Light liquid used as anaesthetic" , "number" .)