Slide unsteadily (7)
I believe the answer is:
slither
'slide unsteadily' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for slither that I've seen before include "Slide, slip" , "Slide unsteadily" , "Quietly advance" , "Move like a snake" , "Slide (like a snake)" .)