River pretty muddy in the middle (5)
I believe the answer is:
tweed
'river' is the definition.
(river in Scotland)
'pretty muddy in the middle' is the wordplay.
'pretty' becomes 'twee' (twee can mean cute or pretty).
'in the middle' indicates the central letters.
The middle of 'muddy' is 'd'.
'twee'+'d'='TWEED'
(Other definitions for tweed that I've seen before include "country fabric" , "Woollen cloth - Scotland/England border river" , "Thick woollen fabric from Scotland" , "Twilled cloth" , "Rough woollen cloth" .)