Declining to accept sources of any really hoary old theatre piece? (3,5)
I believe the answer is:
war horse
'old theatre piece?' is the definition.
(war horse can mean a frequently performed play)
'declining to accept sources of any really hoary' is the wordplay.
'declining' becomes 'worse' (declining means getting worse).
'to accept' is an insertion indicator.
'sources of' says to take the initial letters (the beginning of a word is its sauce).
The first letters of 'any really hoary' is 'arh'.
'worse' placed around 'arh' is 'WAR-HORSE'.
(Other definitions for war horse that I've seen before include "West End production" , "play" .)