A common mathematical expression? (6,8)
I believe the answer is:
vulgar fraction
'expression?' is the definition.
(I have seen 'Mathematical expression' mean 'vulgar fraction' so perhaps 'expression' could also mean 'vulgar fraction')
'a common mathematical' is the wordplay.
'a common' becomes 'vulgar' (I've seen this before).
'mathematical' becomes 'fraction' (I can't explain this - if you can you should give a lot more credence to this answer).
'vulgar'+'fraction'='VULGAR FRACTION'
Can you help me to learn more?
(Other definitions for vulgar fraction that I've seen before include "Mathematical expression" , "in bar between five and six, for example" , "in which division is evident" .)