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

I believe the answer is:

seeing fit

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

'on witnessing the spasm' is the wordplay.
'on' means one lot of letters go next to another.
'witnessing' becomes 'seeing' ('see' can be a synonym of 'witness').
'the spasm' becomes 'fit' (I've seen this before).
'seeing' put next to 'fit' is 'SEEING FIT'.

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