Lacking, but not for long (5)
I believe the answer is:
short
'lacking' is the definition.
(to be short something is to lack it)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for short that I've seen before include "Of small duration" , "Literary genre such as those by ..." , "Limited in duration" , "Neat" , "Not long enough" .)