So as to surprise (8)
I believe the answer is:
unawares
'so as to surprise' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for unawares that I've seen before include "Unknowingly - by surprise" , "Be taken thus if off-guard" , "Not expecting" , "Unexpectedly, without noticing in new auras" , "New auras may be unexpected" .)