Move slowly in extremity (4)
I believe the answer is:
edge
I believe this is a double definition.
'move slowly' is the first definition.
(I've seen this before)
'extremity' is the second definition.
(edge is a kind of extremity)
'in' acts as a link.
(Other definitions for edge that I've seen before include "Surface boundary" , "Upper hand" , "small advantage" , "Move furtively; intense quality" , "Sharp ridge" .)