Terribly stupid about, the former would have it (10)

I believe the answer is:

stipulated

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

'would have it' is the definition.
I can't judge whether this definition defines the answer.

'terribly stupid about the former' is the wordplay.
'terribly' is an anagram indicator.
'about' is an insertion indicator.
'the former' becomes 'late' (late can mean former or previous).
'stupid' anagrammed gives 'stipud'.
'stipud' placed around 'late' is 'STIPULATED'.

Can you help me to learn more?

(Another definition for stipulated that I've seen is " Required, specified".)

I've seen this clue in the Evening Standard.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.