Wanted to catch teacher in the act! (7)
I believe the answer is:
desired
'wanted' is the definition.
(similar in meaning)
'teacher in the act' is the wordplay.
'teacher' becomes 'sir' (a male teacher might be addressed as 'sir').
'in the' indicates putting letters inside.
'act' becomes 'deed' (deed is a kind of act).
'sir' going inside 'deed' is 'DESIRED'.
'to catch' acts as a link.
(Other definitions for desired that I've seen before include "Sought after" , "Wanted greatly" , "Craved" , "requested" , "much wished for" .)