In which I decided to reprimand (5)
I believe the answer is:
chide
'to reprimand' is the definition.
(synonyms)
'in which i decided' is the wordplay.
'in' says the answer is hidden in the clue.
'CHIDE' can be found hidden inside 'which i decided'.
(Other definitions for chide that I've seen before include "Censure angrily" , "lecture" , "Seeing scold" , "Rebuke, scold" , "Santa's laugh" .)