Rue being right over heron (6)
I believe the answer is:
regret
'rue' is the definition.
(I know that rue can be written as regret)
'right over heron' is the wordplay.
'right' becomes 'r' (common abbreviation).
'over' means one lot of letters go next to another (in a down clue, some letters go over others).
'heron' becomes 'egret' (egret is a kind of heron).
'r'+'egret'='REGRET'
'being' is the link.
(Other definitions for regret that I've seen before include "Wish undone" , "Feel sad or disappointed about" , "Mourn" , "Contrition" , "Lament" .)