Legislator's work ultimately to stop malware spreading (8)
I believe the answer is:
lawmaker
'legislator's' is the definition.
(legislators make laws)
'work ultimately to stop malware spreading' is the wordplay.
'ultimately' suggests the final letters.
'to stop' means one lot of letters goes inside another (stopping can mean blocking a gap).
'spreading' is an anagram indicator.
The last letter of 'work' is 'k'.
'malware' anagrammed gives 'lawmaer'.
'k' inserted into 'lawmaer' is 'LAWMAKER'.
(Other definitions for lawmaker that I've seen before include "sitting in Parliament?" , "Legislator" , "Sod, possibly" .)