Enlarging house for estranged lover? Thickhead conclusion! (9)

I believe the answer is:

extending

I'm a little stuck... Click here to teach me more about this clue!

'enlarging house' is the definition.
I can't tell whether this defines the answer.

'estranged lover? thickhead conclusion' is the wordplay.
'estranged lover? thickhead' becomes 'ext' (I can't explain this - if you can you should give a lot more credence to this answer).
'conclusion' becomes 'ending' (synonyms).
'ext'+'ending'='EXTENDING'

'for' is the link.

Can you help me to learn more?

(Other definitions for extending that I've seen before include "carrying on" , "Stretching out over a distance" , "Adding to" .)

I've seen this clue in the Sydney Morning Herald.

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