Unpleasant flats suited eccentric (11)
I believe the answer is:
distasteful
'unpleasant' is the definition.
(I've seen this before)
'flats suited eccentric' is the wordplay.
'eccentric' is an anagram indicator.
'flats'+'suited'='flatssuited'
'flatssuited' is an anagram of 'DISTASTEFUL'.
(Other definitions for distasteful that I've seen before include "Unpalatable" , "Unpleasant" , "offensive" .)