Painter of waterfowl around the head of the river (5)
I believe the answer is:
corot
'painter' is the definition.
(thesaurus)
'waterfowl around the head of the river' is the wordplay.
'waterfowl' becomes 'coot' (I've seen this before).
'around' is an insertion indicator.
'the head of' indicates taking the first letters.
The initial letter of 'river' is 'r'.
'coot' going around 'r' is 'COROT'.
'of' acts as a link.
(Other definitions for corot that I've seen before include "18c French painter" , "Barbizon School artist" , "French artist" , "C19 French landscape painter" , "French landscape painter, 1796-1875" .)