Many making mouse run unexpectedly (8)
I believe the answer is:
numerous
'many' is the definition.
(similar in meaning)
'mouse run unexpectedly' is the wordplay.
'unexpectedly' indicates anagramming the letters.
'mouse'+'run'='mouserun'
'mouserun' anagrammed gives 'NUMEROUS'.
'making' is the link.
(Other definitions for numerous that I've seen before include "Goodly but indefinite number" , "There's no shortage" , "Great in number, many" , "Amounting to a large indefinite number" , "Profuse" .)