Greeting former pupil inside with new reinforcement (7)
I believe the answer is:
hobnail
'reinforcement' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'greeting former pupil inside with new' is the wordplay.
'greeting' becomes 'hail' ('hail' is a greeting).
'former pupil' becomes 'OB' (abbreviation for 'Old Boy').
'inside' means one lot of letters goes inside another.
'with' is an insertion indicator.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'hail' going around 'ob' is 'hobail'.
'hobail' placed around 'n' is 'HOBNAIL'.
Can you help me to learn more?
(Other definitions for hobnail that I've seen before include "Stud for boots" , "Protection for the sole of a heavy boot" , "Walk over roughly" , "One keeping shoes on" , "Stud on boot" .)