Rook in front of tree is not considered (4)
I believe the answer is:
rash
'not considered' is the definition.
(I've seen this before)
'rook in front of tree' is the wordplay.
'rook' becomes 'r' (abbreviation in chess).
'in front of' says to put letters next to each other.
'tree' becomes 'ash' (ash is a kind of tree).
'r'+'ash'='RASH'
'is' is the link.
(Other definitions for rash that I've seen before include "Daredevil" , "Red area on skin" , "Ill-considered - outbreak" , "Hasty; series of things" , "spate" .)