Nasty rat is moving around (5)
I believe the answer is:
astir
'moving around' is the definition.
(I've seen this in another clue)
'nasty rat is' is the wordplay.
'nasty' indicates an anagram.
'rat'+'is'='ratis'
'ratis' is an anagram of 'ASTIR'.
(Other definitions for astir that I've seen before include "Excitedly moving" , "Moving; out of bed" , "This isn't still" , "In motion - out of bed" , "Up and about -- is art (anag)" .)