Bellow at rower behind Head of the River (4)
I believe the answer is:
roar
'bellow' is the definition.
(I know that bellow can be written as roar)
'rower behind head of the river' is the wordplay.
'rower' becomes 'oar' (I've seen this in another clue).
'behind' means one lot of letters go next to another.
'head of' says to take the initial letters.
The initial letter of 'river' is 'r'.
'oar' put after 'r' is 'ROAR'.
'at' is the link.
(Other definitions for roar that I've seen before include "Deep loud sound as of lion or bull" , "Loud cry" , "Loud noise of angry bull" , "The __ of the Greasepaint -- The Smell of the Crowd (1964 musical)" , "Make a loud noise" .)