Sanctimonious in bed raising subject for argument (5)
I believe the answer is:
topic
'subject for argument' is the definition.
(topic for discussion)
'sanctimonious in bed raising' is the wordplay.
'sanctimonious' becomes 'pi' (British abbreviation for pious).
'in' means one lot of letters goes inside another.
'bed' becomes 'cot' (cot is a type of bed).
'raising' is a reversal indicator (in a down clue, letters go up).
'cot' reversed gives 'toc'.
'pi' going into 'toc' is 'TOPIC'.
(Other definitions for topic that I've seen before include "Subject (of speech, etc)" , "A subject for conversation or debate" , "Subject (for conversation, etc)" , "Subject matter of a conversation or discussion" , "Something to discuss" .)