Unexpected thread shortage (6)
I believe the answer is:
dearth
'shortage' is the definition.
(dearth can mean a shortage or lack)
'unexpected thread' is the wordplay.
'unexpected' is an anagram indicator.
'thread' is an anagram of 'DEARTH'.
(Other definitions for dearth that I've seen before include "want" , "Scarcity of thread" , "Severe lack" , "Shortage, lack" , "Scarceness or lack" .)