About to conceal rebuke (5)
I believe the answer is:
chide
'rebuke' is the definition.
('chide' can be a synonym of 'rebuke')
'about to conceal' is the wordplay.
'about' becomes 'c' (abbreviation for 'circa').
'to conceal' becomes 'hide' ('hide' can be a synonym of 'conceal').
'c'+'hide'='CHIDE'
(Other definitions for chide that I've seen before include "Rebuke, scold" , "Seeing scold" , "Reprove" , "Censure angrily" , "Santa's laugh" .)