Perform without notes from Brideshead librettist (2-3)
I believe the answer is:
ad-lib
'perform without notes' is the definition.
(to perform without notes or preparation)
'from brideshead librettist' is the wordplay.
'from' indicates a hidden word.
'AD-LIB' can be found hidden inside 'brideshead librettist'.
(Other definitions for ad-lib that I've seen before include "Speak off the cuff" , "Speak without preparation" , "Extemporise" , "Off the cuff remark" , "spontaneous comment" .)