Disappointment expressed when pet has passed away (7)
I believe the answer is:
doggone
'disappointment expressed' is the definition.
I can't judge whether this defines the answer.
'pet has passed away' is the wordplay.
'pet' becomes 'dog'.
'has' is a charade indicator (letters next to each other).
'passed away' becomes 'gone' ('go' can be a synonym of 'pass away').
'dog'+'gone'='DOGGONE'
'when' is the link.
Can you help me to learn more?
(Other definitions for doggone that I've seen before include "Infernal" , "expression used by irritated person" .)