Shapes made of pine secured by former students (7)
I believe the answer is:
oblongs
'shapes' is the definition.
(oblong is a kind of shape)
'of pine secured by former students' is the wordplay.
'of' becomes 'o''.
'pine' becomes 'long' (to pine for something is to long for it).
'secured by' means one lot of letters goes inside another (inserted letters are held or secured).
'former students' becomes 'bs' (I can't explain this - if you can you should give a lot more credence to this answer).
'long' put into 'bs' is 'blongs'.
'o'+'blongs'='OBLONGS'
'made' is the link.
Can you help me to learn more?
(Another definition for oblongs that I've seen is " Rectangles".)