Void and somewhat ill-understood rejection (4)
I believe the answer is:
null
'void' is the definition.
(synonyms)
'somewhat ill-understood rejection' is the wordplay.
'somewhat' says the answer is hidden in the clue.
'rejection' is a reversal indicator (to reject is to send back).
'llun' is hidden within 'ill understood'.
'llun' backwards is 'NULL'.
'and' is the link.
(Other definitions for null that I've seen before include "Without legal force" , "Not valid" , "Useless - nothing" , "Non-existent and void" , "Legally void" .)