ST Cryptic 2640

[jenniferd]

For 28a "Source of electricity that prisoner would like" I have put DRY-Cell.
21d Like first or second disturbance disrupting exam (7) O???N?L The L comes from second L of 28a answer. Is it ORDINAL and why?

[MarkRae]

Ordinal = first, second, third etc, as opposed to one, two, three etc which are cardinal.

DIN in ORAL

[ulysses100]

first and second are ordinal numbers
disturbance = din
exam = oral

[jenniferd]

Thanks Both. That was really THICK of me.
Could you confirm that 24d Exchange, by letters (5) is BANDY. B and Y

[boxtops]

yes B and Y

[jenniferd]

Can't give everyone best answer so thanks to all again.