Garment rook found inside a tree (6)
I believe the answer is:
anorak
'garment' is the definition.
(anorak is a kind of garment)
'rook found inside a tree' is the wordplay.
'rook' becomes 'R' (chess abbreviation).
'found inside' indicates putting letters inside.
'a' becomes 'an'.
'tree' becomes 'oak' (oak is a type of tree).
'an'+'oak'='anoak'
'r' inserted within 'anoak' is 'ANORAK'.
(Other definitions for anorak that I've seen before include "Trivia obsessive" , "Weather-resistant fabric" , "Jacket for outdoors" , "Casual jacket with hood and zip" , "Garment; studious person" .)