River in which star is reflected (4)
I believe the answer is:
avon
'river' is the definition.
(I've seen this before)
'star is reflected' is the wordplay.
'star' becomes 'nova' (nova is a kind of star).
'is reflected' says the letters should be written backwards.
'nova' reversed gives 'AVON'.
'in which' is the link.
(Other definitions for avon that I've seen before include "River and former county" , "Shakespeare its swan" , "River running through Bath" , "River flowing through Bristol to the Severn" , "Bath's river" .)