Top tree was rotten (7)
I believe the answer is:
sweater
'top' is the definition.
(type of top or jumper)
'tree was rotten' is the wordplay.
'rotten' indicates an anagram.
'tree'+'was'='treewas'
'treewas' is an anagram of 'SWEATER'.
(Other definitions for sweater that I've seen before include "Long-sleeved pullover" , "Warm garment" , "Top" , "Re waste: it's a garment" , "mugger once" .)