There may 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 may be' is the wordplay.
'may be' indicates an anagram.
'there' with letters rearranged gives 'THREE'.
(Other definitions for three that I've seen before include "Start to count back, briefly" , "A crowd, you might say" , "Prime number" , "Sixth root of seven hundred and twenty nine" , "Feet in 3 Down" .)