Pius etc must keep holy symbols of remembrance (7)
I believe the answer is:
poppies
'symbols of remembrance' is the definition.
(Remembrance Day poppies)
'pius etc must keep holy' is the wordplay.
'pius etc' becomes 'popes' (several popes have been called Pius).
'must keep' means one lot of letters goes inside another.
'holy' becomes 'pi' (pi can mean pious).
'popes' going around 'pi' is 'POPPIES'.
(Other definitions for poppies that I've seen before include "Scarlet flowers" , "Large bright flowers" , "Opium-yielding flower plants" , "Sources of opium" .)