That deserter Edward was treacherous (6)
I believe the answer is:
'treacherous' is the definition.
The answer and definition are different parts of speech. However, past participle verbs and adjectives occasionally mean the same thing.
'that deserter edward' is the wordplay.
'that deserter' becomes 'rat' (both can mean a traitor).
'edward' becomes 'ted'.
'was' is the link.
Can you help me to learn more?
(Other definitions for ratted that I've seen before include "Deserted the side, rodent-like" , "Betrayed or informed, the rodent" , "Broke promise in treacherous or rodent style" , "Betrayed like a rodent" , "Deserted; very drunk" .)