Moving north - that's sharp (5)
I believe the answer is:
thorn
'that's sharp' is the definition.
(sharp part of a plant)
'moving north' is the wordplay.
'moving' indicates an anagram.
'north' with letters rearranged gives 'THORN'.
(Other definitions for thorn that I've seen before include "It causes irritation and annoyance" , "Source of irritation" , "(Rose's) prickle" , "Sharp-pointed tip on a stem" , "tip of R [ROSE]" .)