Thrilled about muck-up? That's harsh! (8)
I believe the answer is:
strident
'harsh' is the definition.
(I've seen this before)
'thrilled about muck-up?' is the wordplay.
'thrilled' becomes 'sent' (send can mean to thrill or excite).
'about' is an insertion indicator.
'muck' becomes 'dirt' (I've seen this before).
'up?' says the letters should be written in reverse (I've seen 'upped' mean this (in down clue, made to go up)).
'dirt' reversed gives 'trid'.
'sent' going around 'trid' is 'STRIDENT'.
'that's' is the link.
(Other definitions for strident that I've seen before include "Loudly assertive" , "Loud and vociferous" , "Loud and harsh over tridents" , "making a lot of noise" , "making loud noises" .)