Right about catching Illyrian fool heading off with old sixpenny piece (6)
I believe the answer is:
tester
'old sixpenny piece' is the definition.
'right about catching illyrian fool heading off' is the wordplay.
'right' becomes 'rt'.
'about' is a reversal indicator.
'catching' means one lot of letters goes inside another.
'illyrian fool' becomes 'feste' (character in Twelfth Night, set in Illyria).
'heading off' means to remove the first letter.
'rt' back-to-front is 'tr'.
'feste' with its initial letter taken off is 'este'.
'tr' placed around 'este' is 'TESTER'.
'with' acts as a link.
(Other definitions for tester that I've seen before include "One pumps" , "covering at night" , "old coin" , "Street (anag.)" , "Kind of examiner or bed canopy" .)