Put on; river (3)
I believe the answer is:
don
This clue is a double definition.
'put on' is the first definition.
(as in donning or putting on an item of clothing)
'river' is the second definition.
(I know that Don is a river)
(Other definitions for don that I've seen before include "and 13 Down: Mozart opera" , "University teacher" , "Spanish gentleman" , "Yorkshire river" , "Professor" .)