Article in class: you look briefly (4,2,3)

'look briefly' is the definition.
(as in casting an eye over something quickly)

'article in class you' is the wordplay.
'article' becomes 'an' (English indefinite article).
'in' indicates putting letters inside.
'class' becomes 'caste' (type of social class).
'you' becomes 'ye' (historical version of 'you').
'an' placed inside 'caste' is 'castane'.
'castane'+'ye'='CAST AN EYE'