Spineless fish extremely lazy about fighting (8)
I believe the answer is:
cowardly
'spineless' is the definition.
(thesaurus)
'fish extremely lazy about fighting' is the wordplay.
'fish' becomes 'cod' (type of fish).
'extremely' suggests removing the centre (only the extremes of the word are used).
'about' is an insertion indicator.
'fighting' becomes 'war' ().
'lazy' with its middle taken out is 'ly'.
'cod'+'ly'='codly'
'codly' going around 'war' is 'COWARDLY'.
(Other definitions for cowardly that I've seen before include "Faint-hearted" , "Lily-livered" , "Lacking courage" , "Yellow" , "chicken" .)