Feel sorry for rook and heron (6)
I believe the answer is:
regret
'feel sorry' is the definition.
(I've seen this before)
'rook and heron' is the wordplay.
'rook' becomes 'R' (chess abbreviation).
'and' means one lot of letters go next to another.
'heron' becomes 'egret' (egret is a kind of heron).
'r'+'egret'='REGRET'
'for' acts as a link.
(Other definitions for regret that I've seen before include "Rue - remorse" , "Be sorry for an action" , "Mourn" , "Rue, repent" , "Deprecate" .)