Tail of rook found in tree (6)
I believe the answer is:
breech
'tail' is the definition.
(thesaurus)
'rook found in tree' is the wordplay.
'rook' becomes 'R' (chess abbreviation).
'found in' means one lot of letters goes inside another.
'tree' becomes 'beech' (beech is a kind of tree).
'r' inserted within 'beech' is 'BREECH'.
'of' acts as a link.
(Other definitions for breech that I've seen before include "One's behind" , "Prat" , "Rear part of rifle" , "Back end of a gun barrel" , "Lower part of a gun" .)