Where one might see students with a learner of a casual sort (8)
I believe the answer is:
informal
'of a casual sort' is the definition.
(as opposed to formal)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for informal that I've seen before include "Casual and unceremonious" , "If normal, be casual and without ceremony" , "Unofficial, relaxed" , "Casual, not official" , "Casual or unofficial" .)