Crossword Genius

What heights the incredibility had reached? (8)

Ross

I believe the answer is:

tallness

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

'incredibility had reached?' is the definition.
I can't judge whether this definition defines the answer.

'what heights the' is the wordplay.
I cannot really understand how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found within the answer.
The remaining letters 'allness' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

Can you help me to learn more?

I've seen this clue in the Evening Standard.
Want a hint initially instead of a full solution? Install my app