Beer supporter necks gallon in maths class (7)
I believe the answer is:
algebra
'class' is the definition.
(I know that algebra is a class)
'beer supporter necks gallon in' is the wordplay.
'beer' becomes 'ale' (type of beer).
'supporter necks' becomes 'bra' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'gallon' becomes 'g' (abbreviation for the unit of liquid).
'in' is an insertion indicator.
'ale'+'bra'='alebra'
'alebra' going around 'g' is 'ALGEBRA'.
'maths' is the link.
This may not be correct. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for algebra that I've seen before include "Maths with letters representing numbers" , "Part of the maths syllabus" , "system employing symbols" , "school subject" , "exercise in school?" .)