Chief support where sea ultimately meets river (8)
I believe the answer is:
mainstay
'chief support' is the definition.
(I've seen this before)
'where sea ultimately meets river' is the wordplay.
'where sea' becomes 'main' ('main' can be a synonym of 'sea'. I am not sure about the 'where' bit.**).
'ultimately' indicates one should take the final letters.
'river' becomes 'tay' (Scottish river).
The final letter of 'meets' is 's'.
'main'+'s'+'tay'='MAINSTAY'
Can you help me to learn more?
(Other definitions for mainstay that I've seen before include "Strong cable that secures a mast, chief support generally" , "Chief support, like mast cable" , "Linchpin, principle support" , "Pillar of strength" , "Piece of rigging" .)