Head away from watercourse's foul odour (4)
I believe the answer is:
reek
'foul odour' is the definition.
'reek' can be an answer for 'odour' (reek is a kind of odour). I am unsure of the 'foul' bit.
'head away from watercourse's' is the wordplay.
'head away from' means to remove the first letter (I've seen 'heading away from' mean this).
'watercourse' becomes 'creek' (creek is a kind of watercourse).
'creek' with its initial letter taken off is 'REEK'.
Can you help me to learn more?
(Other definitions for reek that I've seen before include "Effluvium" , "bad odour" , "smoke in Balmoral" , "Pong" , "Smell pungently" .)