There could be some musketeers (5)
I believe the answer is:
'there could be some musketeers' is the definition.
I can't tell whether this definition defines the answer.
'there could be' is the wordplay.
'could be' indicates anagramming the letters.
'there' anagrammed gives 'THREE'.
Can you help me to learn more?
(Other definitions for three that I've seen before include "Trinity, triad" , "Number of times N appears in this completed grid" , "A crowd, you might say" , "Snooker score for potting green ball" , "Number of Dumas's musketeers" .)