River in Slough bearing bracken? (4)
I believe the answer is:
fern
'bracken?' is the definition.
(bracken is a kind of fern)
'river in slough' is the wordplay.
'river' becomes 'r'.
'in' is an insertion indicator.
'slough' becomes 'fen' (I've seen this before).
'r' inserted inside 'fen' is 'FERN'.
'bearing' acts as a link.
(Other definitions for fern that I've seen before include "Flowerless plant with feathery fronds" , "Non-flowering plant" , "Feathery frond plant" , "Plant with featherlike leaves" , "Leafy plant" .)