Being regularly culled is important (3)
I believe the answer is:
big
'important' is the definition.
(synonyms)
'being regularly culled' is the wordplay.
'regularly culled' means one should take alternating letters (letters removed at regular intervals).
The alternate letters of 'being' are 'BIG'.
'is' acts as a link.
(Other definitions for big that I've seen before include "Sizeable" , "Massive" , "Important" , "Circus tent" , "oversized" .)