Battered cod has head of bream alongside (6)
I believe the answer is:
shabby
'battered' is the definition.
(thesaurus)
'cod has head of bream alongside' is the wordplay.
'cod' indicates an anagram (cod can mean fake or inauthentic).
'head of' says to take the initial letters.
'alongside' becomes 'by' (I've seen this in other clues).
The initial letter of 'bream' is 'b'.
'has' is an anagram of 'sha'.
'sha'+'b'+'by'='SHABBY'
(Other definitions for shabby that I've seen before include "Threadbare - dingy - mean - base" , "Ragged - mean" , "Badly worn" , "Ill-dressed - mean" , "'Scruffy, tattered (6)'" .)