Bubble on skin (7)
I believe the answer is:
blister
'bubble on skin' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for blister that I've seen before include "Thin bubble on the skin" , "Friction bubble on skin" , "Sore bubble on skin" , "Bubble-like swelling on skin" , "Effect of rubbing on skin" .)