Shock when pair climbing spotted mount (9)

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

APPAL(to shock/horrify) + reversal of ( … climbing , in a down clue) [ AS(when/at that particular moment, as in “as you enter”) + OO(I’m not sure about this: letters representing a pair of eyes? Spectacles? Breasts? Or is it just a pair of letters) ].