Pay attention to river rising below hospital (4)
I believe the answer is:
heed
'pay attention to' is the definition.
(thesaurus)
'river rising below hospital' is the wordplay.
'river' becomes 'dee' (River Dee**).
'rising' says the letters should be written in reverse (going up in a down clue).
'below' means one lot of letters go next to another.
'hospital' becomes 'h'.
'dee' reversed gives 'eed'.
'eed' after 'h' is 'HEED'.
(Other definitions for heed that I've seen before include "Mark" , "Pay close attention to" , "Regard" , "Be mindful of" , "Take note of" .)