Relish playing part in rehearsal address in Greenwich (5,8)
I believe the answer is:
salad dressing
'relish' is the definition.
(type of sauce or relish)
'playing part in rehearsal address in greenwich' is the wordplay.
'playing part in' says the answer is hidden in the clue.
'SALAD-DRESSING' is hidden within 'rehearsal address in greenwich'.
(Other definitions for salad dressing that I've seen before include "what goes on rocket?" , "leaves having taken this coat?" , "saucy thing!" , "Mayonnaise, e.g." , "Eg, Thousand Island or French" .)