Thinking it right, on witnessing the spasm (6,3)

I believe the answer is:

seeing fit

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

'thinking it right' is the definition.
I can't judge whether this defines the answer.

'on witnessing the spasm' is the wordplay.
'on' says to put letters next to each other.
'witnessing' becomes 'seeing' ('see' can be a synonym of 'witness').
'the spasm' becomes 'fit' (I've seen this before).
'seeing'+'fit' is 'SEEING FIT'.

Can you help me to learn more?

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.