Medical sticker found on wall (7)

I believe the answer is:

plaster

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

'medical sticker' is the definition.
'plaster' can be an answer for 'sticker' (thesaurus). I am not certain of the 'medical' bit.

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for plaster that I've seen before include "Wall surface" , "Adhesive material" , "Smear" , "Strip of material for dressing wound" , "Wall finish - protection for cut" .)

I've seen this clue in The Mirror.

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