Rather messy mat – whose? (8)
I believe the answer is:
somewhat
'rather' is the definition.
(I've seen this before)
'messy mat whose?' is the wordplay.
'messy' is an anagram indicator.
'mat'+'whose'='matwhose'
'matwhose' anagrammed gives 'SOMEWHAT'.
(Other definitions for somewhat that I've seen before include "To some degree, sort of" , "Rather, a little" , "To a moderate extent" , "To an extent, a little" , "To a certain degree - a bit" .)