How criminal is clapped when apprehended! (2,5)

I believe the answer is:

in irons

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

'how criminal is clapped' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.

'when apprehended' is the wordplay.
I cannot really understand how this works, but
'when' could be 'on' (I've seen this in other clues) and 'on' is present in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

I've seen this clue in The Irish Times.

Want a hint initially instead of a full solution? Install my app