Mysterious giant mice getting agitated (9)
I believe the answer is:
enigmatic
'mysterious' is the definition.
(I've seen this in another clue)
'giant mice getting agitated' is the wordplay.
'getting agitated' indicates anagramming the letters.
'giant'+'mice'='giantmice'
'giantmice' is an anagram of 'ENIGMATIC'.
(Other definitions for enigmatic that I've seen before include "Mysterious or puzzling, like Mona Lisa's smile" , "not obvious" , "Obscure, puzzling" , "Puzzling, like the Sphinx" , "Ambiguous or puzzling" .)