Only small parts of the course of a river (4)
I believe the answer is:
tees
'river' is the definition.
(River Tees)
'only small parts of the course of a' is the wordplay.
I cannot quite see how this works, but
'small' could be 's' (abbreviation - e.g. clothes size) and 's' is found within the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is located in the answer.
'course' could be 'e' and 'e' is present in the answer.
A single letter 'e' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for tees that I've seen before include "banker" , "English river" , "Golf pegs" , "used by golfers" , "Stockton's river" .)