Shy about fighting heartless duel, being faint-hearted (8)
I believe the answer is:
cowardly
'faint-hearted' is the definition.
(faint-hearted means cowardly or not very brave)
'shy about fighting heartless duel' is the wordplay.
'shy' becomes 'coy' (coy can mean shy or modest).
'about' means one lot of letters goes inside another.
'fighting' becomes 'war'.
'heartless' suggests removing the centre.
'duel' with its centre taken out is 'dl'.
'war'+'dl'='wardl'
'coy' enclosing 'wardl' is 'COWARDLY'.
'being' is the link.
(Other definitions for cowardly that I've seen before include "Faint-hearted" , "Lacking courage" , "Lily-livered" , "Yellow" , "chicken" .)