Polish river comes first as major river (5)
I believe the answer is:
rhone
'major river' is the definition.
'rhone' can be an answer for 'river' (Rhone is an example). I'm not sure about the 'major' bit.
'polish river comes first' is the wordplay.
'polish' becomes 'hone' (I've seen this before).
'river' becomes 'r' (abbreviation).
'comes first' means one lot of letters go next to another.
'hone' after 'r' is 'RHONE'.
'as' acts as a link.
Can you help me to learn more?
(Other definitions for rhone that I've seen before include "River at Lyons" , "Swiss-French river" , "Major French river, rising in Switzerland" , "Fr. river and wine" , "12 wine region" .)