Westbound vessel enters Volga regularly – and a Czech river (6)
I believe the answer is:
vltava
'a czech river' is the definition.
(I've seen this before)
'westbound vessel enters volga regularly' is the wordplay.
'westbound' says the letters should be written backwards (letters written from right to left).
'vessel' becomes 'vat' (vat is a kind of vessel**).
'enters' is an insertion indicator.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternate letters of 'volga' are 'vla'.
'vat' written backwards gives 'tav'.
'tav' put into 'vla' is 'VLTAVA'.
'and' is the link.
(Other definitions for vltava that I've seen before include "River at Prague portrayed musically by Smetana" , "banker in Europe" , "Czech river" .)