Lukewarm, having only one ventricle? (11)

I believe the answer is:

halfhearted

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

'lukewarm' is the definition.
(synonyms)

'only one ventricle?' is the wordplay.
I cannot quite understand how this works, but
'one' could be 'a' (a thing is one thing) and 'a' is located in the answer.
'ventricle?' could be 'heart' (synonyms) and 'heart' is located in the answer.
This explanation may well be incorrect...

'having' acts as a link.

Can you help me to learn more?

I've seen this clue in The Daily Express.

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