Self-centredness, say, is in order (6)
I believe the answer is:
egoism
'self-centredness' is the definition.
(I've seen this before)
'say is in order' is the wordplay.
'say' becomes 'eg'.
'in' means one lot of letters goes inside another.
'order' becomes 'om' (order of merit).
'is' placed inside 'om' is 'oism'.
'eg'+'oism'='EGOISM'
(Other definitions for egoism that I've seen before include "Self-seeking" , "sense of superiority" , "Self-centredness, vanity" , "Excessive self-regard" , "Exaggerated sense of one's own importance" .)