Neighbour on vacation initially lying back beside a river (4)
I believe the answer is:
avon
'river' is the definition.
(I know that Avon is a river)
'neighbour on vacation initially lying back beside a' is the wordplay.
'initially' suggests taking the first letters.
'lying back' shows that the letters should be reversed in order.
'beside' says to put letters next to each other (I've seen this in other clues).
The initial letters of 'neighbour on vacation' is 'nov'.
'nov' written backwards gives 'von'.
'von' put after 'a' is 'AVON'.
(Other definitions for avon that I've seen before include "River of the Bard" , "Name of several English rivers" , "River flowing through Shakespeare's Stratford" , "English river flowing through Stratford" , "River at Shakespeare's Warwickshire birthplace" .)