Clashing with Troy, except if sweetheart intervenes (8)
I believe the answer is:
tuneless
'clashing' is the definition.
(clashing musically)
'troy except if sweetheart intervenes' is the wordplay.
'troy' becomes 't'.
'except if' becomes 'unless' ('unless' can mean 'except if...').
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'intervenes' means one lot of letters goes inside another.
'unless' enclosing 'e' is 'uneless'.
't'+'uneless'='TUNELESS'
'with' acts as a link.
(Other definitions for tuneless that I've seen before include "Not melodious" , "Unmelodious" , "spoiled by lack of air?" , "Like some whistling" , "harsh" .)