Dull crimson was important (8)
I believe the answer is:
mattered
'was important' is the definition.
(to matter is to be important)
'dull crimson' is the wordplay.
'dull' becomes 'matte' (similar in meaning).
'crimson' becomes 'red' (synonyms).
'matte'+'red'='MATTERED'
(Other definitions for mattered that I've seen before include "Was significant" , "Counted, had significance" , "Was of importance" , "Counted, had weight" .)