Left at four, getting back being very important (5)
I believe the answer is:
vital
'very important' is the definition.
(I've seen this in another clue)
'left at four getting back being' is the wordplay.
'left' is a reversal indicator.
'four' becomes 'IV' (Roman numerals for four).
'getting' says to put letters next to each other.
'back being' becomes 'l' (I can't justify this - if you can you should believe this answer much more).
'at'+'iv'='ativ'
'ativ' in reverse letter order is 'vita'.
'vita'+'l'='VITAL'
Can you help me to learn more?
(Other definitions for vital that I've seen before include "Imperative" , "Critical, necessary" , "Crucial" , "Essential; lively" , "Most important, like certain body organs" .)