Mask thief fashioned that's rudimentary (9)
I believe the answer is:
makeshift
'rudimentary' is the definition.
(makeshift can mean improvised or rudimentary)
'mask thief fashioned' is the wordplay.
'fashioned' indicates an anagram.
'mask'+'thief'='maskthief'
'maskthief' with letters rearranged gives 'MAKESHIFT'.
'that's' is the link.
(Other definitions for makeshift that I've seen before include "crude arrangement" , "Interim, temporary" , "temporary measure" , "this'll do for now" , "not meant to last" .)