Most shy Mitt dies horribly (8)
I believe the answer is:
timidest
'most shy' is the definition.
('timid' can be similar in meaning to 'shy')
'mitt dies horribly' is the wordplay.
'horribly' is an anagram indicator.
'mitt'+'dies'='mittdies'
'mittdies' with letters rearranged gives 'TIMIDEST'.
(Other definitions for timidest that I've seen before include "most likely to shrink?" , "Least confident" .)