Note most of river Thames’s source in county (8)
I believe the answer is:
somerset
'county' is the definition.
(I know that Somerset is a county)
'note most of river thames's source' is the wordplay.
'note' becomes 'so' (musical note).
'most of river' becomes 'merse' (I am not sure about this - if you are sure you should believe this answer much more).
'source' suggests taking the first letters (source can mean a beginning or origin).
The first letter of 'thamess' is 't'.
'so'+'merse'+'t'='SOMERSET'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for somerset that I've seen before include "part of the west" , "head over heels" , "Street location" , "... and another" , "Cheddar Gorge's county" .)