Mysterious mutation of giant mice (9)
I believe the answer is:
enigmatic
'mysterious' is the definition.
(I've seen this before)
'mutation of giant mice' is the wordplay.
'mutation of' indicates anagramming the letters (I've seen 'mutation' mean this).
'giant'+'mice'='giantmice'
'giantmice' is an anagram of 'ENIGMATIC'.
(Other definitions for enigmatic that I've seen before include "Cryptic, mysterious" , "Puzzling, like the Sphinx" , "Mystifying" , "with mysterious substance" , "not obvious" .)