Justify a record in composure (9)
I believe the answer is:
apologise
'justify' is the definition.
(synonyms)
'a record in composure' is the wordplay.
'record' becomes 'log' (log is a kind of record).
'in' means one lot of letters goes inside another.
'composure' becomes 'poise' (poise is a kind of composure).
'log' placed within 'poise' is 'pologise'.
'a'+'pologise'='APOLOGISE'
(Other definitions for apologise that I've seen before include "Express regret for a wrong (S, not Z)" , "Recognise fault" , "Show regret" , "Say sorry, my mistake" , "Say one is sorry" .)