...in lean monarch's absence, naturally (7,8)

'in' is the definition.
Both the answer and definition are adverbs. Maybe they are linked in a way I don't understand?

'lean monarch's absence naturally' is the wordplay.
I cannot really see how this works, but
'lean' could be 'thin' (synonyms) and 'thin' is found within the answer.
'monarch' could be 'king' (king is a kind of monarch) and 'king' is found within the answer.
The remaining letters 'without' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?