Tick off pair I put in detention (9)
I believe the answer is:
reprimand
'tick off' is the definition.
(thesaurus)
'pair i put in detention' is the wordplay.
'pair' becomes 'pr'.
'put in' indicates putting letters inside.
'detention' becomes 'remand'.
'pr'+'i'='pri'
'pri' put inside 'remand' is 'REPRIMAND'.
(Other definitions for reprimand that I've seen before include "Formal rebuke" , "telling off" , "Tick off - carpet" , "talking-to" , "Formal censure" .)