Pay attention, River Humber's opening up (4)
I believe the answer is:
heed
'pay attention' is the definition.
(thesaurus)
'river humber's opening up' is the wordplay.
'river' becomes 'dee' (River Dee**).
'opening' says to take the initial letters.
'up' says the letters should be written backwards (in down clue: letters go upwards).
The initial letter of 'humbers' is 'h'.
'dee'+'h'='deeh'
'deeh' back-to-front is 'HEED'.
(Other definitions for heed that I've seen before include "Take notice of" , "Attend to, mind" , "Pay close attention to" , "Take account of" , "Consider" .)