Go quickly, as crab might? (3)
I believe the answer is:
nip
This clue is a double definition.
'go quickly' is the first definition.
(to nip to a place is to go there quickly)
'as crab might?' is the second definition.
(a crab might nip you with its pincers)
(Other definitions for nip that I've seen before include "Touch of frost" , "Pinch sharply, perhaps in the bud" , "Small drink or small bite" , "Catch" , "Feeling cold" .)