Chicken and fish turning green inside (6)
I believe the answer is:
coward
'chicken' is the definition.
(similar in meaning)
'fish turning green inside' is the wordplay.
'fish' becomes 'cod' (cod is a kind of fish).
'turning' says the letters should be written in reverse.
'green' becomes 'raw' (synonyms).
'inside' means one lot of letters goes inside another.
'raw' back-to-front is 'war'.
'cod' enclosing 'war' is 'COWARD'.
'and' is the link.
(Other definitions for coward that I've seen before include "Person lacking courage" , "Gutless type" , "Theatrical fellow" , "Person who shows fear or timidity" , "writer and composer" .)