Place for nuns and one on diet? (10)
I believe the answer is:
convention
'diet?' is the definition.
(both can mean a parliament or assembly)
'place for nuns and one on' is the wordplay.
'place for nuns' becomes 'convent' (I've seen this before).
'and' is a charade indicator (letters next to each other).
'one' becomes 'i' (Roman numeral).
'convent'+'i'+'on'='CONVENTION'
(Other definitions for convention that I've seen before include "Compact" , "assembly" , "normal practice" , "See 14" , "See 15" .)