Tuna cooked again for character (6)
I believe the answer is:
nature
'for character' is the definition.
(I've seen this before)
'tuna cooked again' is the wordplay.
'cooked' indicates anagramming the letters (letters cooked into a new form).
'again' becomes 're' (prefix meaning 'again').
'tuna' anagrammed gives 'natu'.
'natu'+'re'='NATURE'
(Other definitions for nature that I've seen before include "Inborn characteristics" , "(Person's) character" , "The external, untouched world" , "Motherly" , "Character - wildlife" .)