Lacked, required (6)
I believe the answer is:
needed
This clue is a double definition.
'lacked' is the first definition.
(I've seen this in another clue)
'required' is the second definition.
(I know that required can be written as needed)
(Other definitions for needed that I've seen before include "Missing" , "Required; sounds like worked (dough)" , "Lacked something essential" , "'Lacked, required (6)'" , "Was in want of" .)