A number out there (5)
I believe the answer is:
three
'a number' is the definition.
(three is a number)
'out there' is the wordplay.
'out' indicates an anagram.
'there' anagrammed gives 'THREE'.
(Other definitions for three that I've seen before include "Number of strikes before you're out" , "One, one, one" , "Square root of nine" , "Cardinal number" , "III" .)