Blur, British group (4)
I believe the answer is:
blot
'blur' is the definition.
(thesaurus)
'british group' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'group' becomes 'lot' (lot is a kind of group**).
'b'+'lot'='BLOT'
(Other definitions for blot that I've seen before include "Smudge of ink" , "Ink mark" , "Spot of ink, paint etc" , "Smudge (on escutcheon?)" , "Stain on 1 across?" .)