Sanitiser is discarded on the counter important for seeing membranes (7)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

SANIT [is] ER with the letters IS discarded, reversed (on the counter). Retina = the layer at the back of the eye on which images are formed = a membrane for seeing. I’m not sure what “important” is for, unless the definition is “important-for-seeing membranes”?