I sat at home inhaling oxygen following, say, heart concern (12)
I believe the answer is:
organisation
'concern' is the definition.
(I've seen this before)
'i sat at home inhaling oxygen following say heart' is the wordplay.
'at home' becomes 'in' ('in' can be similar in meaning to 'at home').
'inhaling' is an insertion indicator.
'oxygen' becomes 'o' (atomic symbol for oxygen).
'following' is a charade indicator (letters next to each other).
'say heart' becomes 'organ' (heart is an example).
'in' enclosing 'o' is 'ion'.
'i'+'sat'+'ion'='isation'
'isation' after 'organ' is 'ORGANISATION'.
(Other definitions for organisation that I've seen before include "concern" , "Systematic approach to tasks" , "Running" , "System" .)