One detained by senior teacher (3)
I believe the answer is:
sir
'teacher' is the definition.
(I've seen this in other clues)
'one detained by senior' is the wordplay.
'one' becomes 'i' (roman numeral).
'detained by' means one lot of letters goes inside another (I've seen this in other clues).
'senior' becomes 'sr' (similar in meaning).
'i' inserted inside 'sr' is 'SIR'.
(Other definitions for sir that I've seen before include "Title of 1 across" , "Way to address gentleman" , "Male title of respect" , "Knight's prefix" , "Title of knight or baronet" .)