Giant mice to be dealt with? That's baffling! (9)
I believe the answer is:
enigmatic
'baffling' is the definition.
(thesaurus)
'giant mice to be dealt with?' is the wordplay.
'to be dealt with?' is an anagram indicator (I've seen 'deal with' mean this (deal with can mean to organise or sort out)).
'giant'+'mice'='giantmice'
'giantmice' anagrammed gives 'ENIGMATIC'.
'that's' acts as a link.
(Other definitions for enigmatic that I've seen before include "Ambiguous or puzzling" , "Puzzling, like the Sphinx" , "Cryptic, mysterious" , "Hard to fathom" , "Like a puzzle" .)