That of a square, always? (4)

I believe the answer is:

area

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

Here is my best explanation:

'that of a square always?' is the definition.
I can't judge whether this definition defines the answer.

'of a square always?' is the wordplay.
'of' indicates the answer is hidden within the clue.
'AREA' can be found hidden inside 'square always'.

Can you help me to learn more?