Expert Peg will secure good student (7)
I believe the answer is:
protege
'student' is the definition.
(I know that protege is a type of student)
'expert peg will secure good' is the wordplay.
'expert' becomes 'pro' (short for professional).
'peg' becomes 'tee' (tee is a kind of peg).
'will secure' means one lot of letters goes inside another (inserted letters are secured).
'good' becomes 'g' (abbreviation).
'tee' placed around 'g' is 'tege'.
'pro'+'tege'='PROTEGE'
(Other definitions for protege that I've seen before include "A protected species" , "One under the patronage of another" , "One taken under someone's wing" , "Pupil; ward" , "Blue-eyed boy" .)