Broadcast for every boring responsible student becoming tense (7,7)
I believe the answer is:
present perfect
'tense' is the definition.
(present perfect is a grammatical tense)
'broadcast for every boring responsible student' is the wordplay.
'broadcast' becomes 'sent' (I've seen this in other clues).
'for every' becomes 'per' ('per' means 'for each' or 'for every').
'boring' means one lot of letters goes inside another (some letters must 'bore' a hole into other letters).
'responsible student' becomes 'prefect' (student given special responsibilities).
'sent'+'per'='sentper'
'sentper' put within 'prefect' is 'PRESENT PERFECT'.
'becoming' is the link.
(Other definitions for present perfect that I've seen before include "writing the briefest of thank you letters?" , "I have left in this?" , "giver is happy to hear this?" , "Tense" .)