Dead body of rook in a group of trees (6)
I believe the answer is:
corpse
'dead body' is the definition.
(corpse is a kind of dead body)
'rook in a group of trees' is the wordplay.
'rook' becomes 'r' (abbreviation in chess).
'in' is an insertion indicator.
'a group of trees' becomes 'copse' (I've seen this before).
'r' put within 'copse' is 'CORPSE'.
'of' is the link.
(Other definitions for corpse that I've seen before include "Roof beam" , "Dead bodies" , "Cadaver, dead person" , "Dead body" .)