School subject taken by the sick? (7)

I believe the answer is:

physics

I'm a little stuck... Click here to teach me more about this clue!

'school subject' is the definition.
(I've seen this before)

I don't understand the remainder of the clue.

(Other definitions for physics that I've seen before include "Purges" , "Branch of science" , "natural science" , "One of the sciences" , "Scientific subject" .)

I've seen this clue in The Globe and Mail.

Want a hint initially instead of a full solution? Install my app