Serious lack (6)
I believe the answer is:
dearth
'serious lack' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for dearth that I've seen before include "Red hat for scarcity" , "Lack of resources" , "Scarceness or lack" , "Scarcity of thread" , "From shortage of food" .)