Awfully dull piece left unfinished — that’s clear (8)
I believe the answer is:
pellucid
'clear' is the definition.
('pellucid' can be similar in meaning to 'clear')
'awfully dull piece left unfinished' is the wordplay.
'awfully' is an anagram indicator.
'left unfinished' means to remove the last letter.
'dull'+'piece'='dullpiece'
'dullpiece' with its final letter taken off is 'dullpiec'.
'dullpiec' with letters rearranged gives 'PELLUCID'.
'that's' is the link.
(Other definitions for pellucid that I've seen before include "Translucently clear" , "Absolutely crystal clear" , "Very clear, totally transparent" , "Translucent" , "Crystal-clear" .)