Online magazines containing not a single subject (9)
I believe the answer is:
economics
'subject' is the definition.
(economics is a kind of subject)
'online magazines containing not a single' is the wordplay.
'online' becomes 'e' (as in e-mail or e-commerce).
'magazines' becomes 'comics' (comic is a kind of magazine).
'containing' is an insertion indicator.
'not a single' becomes 'no' (in a phrase like 'no man is an island').
'comics' placed around 'no' is 'conomics'.
'e'+'conomics'='ECONOMICS'
(Other definitions for economics that I've seen before include "Monetary management" , "Study of the goods and services of a country" , "One of the social sciences" , "The study of the finances of a nation" , "Thomas Carlyle's "dismal science"" .)