Mature about Liberal supporter for subject of class (7)
I believe the answer is:
algebra
'subject of class' is the definition.
(as in an algebra class)
'mature about liberal supporter' is the wordplay.
'mature' becomes 'age' (synonyms).
'about' is an insertion indicator.
'liberal' becomes 'l' (abbreviation).
'supporter' becomes 'bra' (I've seen this before).
'age' placed around 'l' is 'alge'.
'alge'+'bra'='ALGEBRA'
'for' acts as a link.
(Other definitions for algebra that I've seen before include "Maths with letters representing numbers" , "for study in class" , "Part of the maths syllabus" , "Branch of mathematics using letters and symbols" , "Calculation by symbols" .)