Rejected battered tuna, new starter in diner (8)
I believe the answer is:
unwanted
'rejected' is the definition.
(similar in meaning)
'battered tuna new starter in diner' is the wordplay.
'battered' indicates an anagram.
'starter in' suggests taking the first letters.
The initial letter of 'diner' is 'd'.
'tuna'+'new'='tunanew'
'tunanew' with letters rearranged gives 'unwante'.
'unwante'+'d'='UNWANTED'
(Other definitions for unwanted that I've seen before include "Not needed" , "redundant" , "Superfluous" , "Outcast" , "Not required" .)