Mull over and reckon to be not at home (5,3)

I believe the answer is:

think out

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'mull over' is the definition.
The definition and answer can be both related to thinking as well as being verbs in their base form.
Perhaps they are linked in a way I don't understand?

'reckon to be not at home' is the wordplay.
'reckon' becomes 'think' (synonyms).
'to be not at home' becomes 'out' (I've seen this before).
'think'+'out'='THINK OUT'

'and' is the link.

Can you help me to learn more?

I've seen this clue in the Evening Standard.

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