There could be a trio (5)
I believe the answer is:
three
'a trio' is the definition.
(I know that trio can be written as three)
'there could be' is the wordplay.
'could be' indicates anagramming the letters.
'there' anagrammed gives 'THREE'.
(Other definitions for three that I've seen before include "Number of times N appears in this completed grid" , "Number of feet in a yard" , "Number that's a crowd" , "Number of Furies or Norns" , "Small number that still makes a crowd?" .)