Hooded jacket Rex found in a tree (6)
I believe the answer is:
anorak
'hooded jacket' is the definition.
(I've seen this before)
'rex found in a tree' is the wordplay.
'rex' becomes 'r' (common abbreviation e.g. GR=George Rex, King Georg).
'found in' indicates putting letters inside.
'a' becomes 'an'.
'tree' becomes 'oak' (oak is a kind of tree).
'an'+'oak'='anoak'
'r' inserted within 'anoak' is 'ANORAK'.
(Other definitions for anorak that I've seen before include "Parka, windcheater" , "Boringly obsessive person" , "Jacket for an obsessive?" , "Hooded waterproof jacket" , "trainspotter" .)