Old order to disperse rhino at intervals with sensitivity (4,3)
I believe the answer is:
'old order to disperse' is the definition.
(legal act used to disperse large groups)
'rhino at intervals with sensitivity' is the wordplay.
'at intervals' means one should take alternating letters (letters are taken at regular intervals).
'with' is a charade indicator (letters next to each other).
'sensitivity' becomes 'tact' (being tactful is being sensitive).
The alternate letters of 'rhino' are 'rio'.
(Other definitions for riot act that I've seen before include "Citator may read this as stern rebuke" , "It's read in the face of disturbances" , "It's read as a warning to disperse" , "One read to the unruly once" , "measure to keep order" .)