That of a monarch with only half a domain? (7)
I believe the answer is:
Here is my best explanation:
'that of a' is the definition.
I can't tell whether this definition defines the answer.
'monarch with only half a domain?' is the wordplay.
'monarch' becomes 'king' (king is a kind of monarch).
'with' is a charade indicator (letters next to each other).
'only half' indicates taking half.
'domain' halved is 'dom'.
Can you help me to learn more?