Old judge's discourse (5)
I believe the answer is:
orate
'discourse' is the definition.
(I've seen this in another clue)
'old judge's' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'judge' becomes 'rate' (rating is a kind of judging).
'o'+'rate'='ORATE'
(Other definitions for orate that I've seen before include "Speak like Cicero?" , "Deliver a speech" , "Pontificate" , "talk at length" , "Speak in public" .)