There could be a number (5)
I believe the answer is:
three
'a number' is the definition.
(three is a kind of number)
'there could be' is the wordplay.
'could be' indicates anagramming the letters.
'there' with letters rearranged gives 'THREE'.
(Other definitions for three that I've seen before include "Sixth root of seven hundred and twenty nine" , "Trinity, triad" , "Number of times N appears in this completed grid" , "Amount in a trio" , "... number of his "Steps to Heaven"" .)