Oath on losing pet? (7)
I believe the answer is:
doggone
'oath on losing pet?' is the definition.
The definition suggests an adjective which matches the answer.
'on losing pet?' is the wordplay.
'on' is a charade indicator (letters next to each other).
'losing' becomes 'gone' (I can't explain this - if you can you should give a lot more credence to this answer).
'pet?' becomes 'dog'.
'gone' after 'dog' is 'DOGGONE'.
Can you help me to learn more?
(Other definitions for doggone that I've seen before include "Infernal" , "expression used by irritated person" .)