Not quick or not quick enough (4)
I believe the answer is:
late
'not quick enough' is the definition.
(I've seen this before)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for late that I've seen before include "Running behind time" , "Far into the night" , "Having bought it" , "After the usual time" , "In arrears" .)