Garment in river, dredged up (3)
I believe the answer is:
mac
'garment' is the definition.
(mac is a kind of garment)
'river dredged up' is the wordplay.
'river' becomes 'cam' (river in Cambridge, England).
'dredged up' says the letters should be written in reverse (in a down clue, letters go up).
'cam' back-to-front is 'MAC'.
'in' acts as a link.
(Other definitions for mac that I've seen before include "one used in shower?" , "A Gaelic son, often in name" , "It may help you keep dry" , "PC" , "Overcoat" .)