A tree was rotting in brine (8)
I believe the answer is:
seawater
'brine' is the definition.
(I know that brine can be written as seawater)
'a tree was rotting' is the wordplay.
'rotting' indicates an anagram.
'a'+'tree'+'was'='atreewas'
'atreewas' anagrammed gives 'SEAWATER'.
'in' acts as a link.
(Other definitions for seawater that I've seen before include "What breaches dyke, perhaps" , "Drink" , "Brine" .)