Frantically busy (6)
I believe the answer is:
hectic
'frantically busy' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for hectic that I've seen before include "Involving feverish activity, flushed" , "'Rushed, madly busy (6)'" , "Involving feverish activity, chaotic" , "particularly busy" , "Frantic or feverish" .)