Be sorry about white heron on river (6)
I believe the answer is:
regret
'be sorry about' is the definition.
(I've seen this before)
'white heron on river' is the wordplay.
'white heron' becomes 'egret' (I've seen this before).
'on' means one lot of letters go next to another.
'river' becomes 'r'.
'egret' after 'r' is 'REGRET'.
(Other definitions for regret that I've seen before include "Feel repentance or disappointment" , "Be sorry for, wish it hadn't happened" , "Lament" , "Feel sad or disappointed about" , "Deprecate" .)