Setter's in favour of half-hearted waffle? Unlikely (10)
I believe the answer is:
improbable
'unlikely' is the definition.
(synonyms)
'setter's in favour of' is the wordplay.
'setter's' becomes 'I'm' (referring to the person who set the crossword).
'in favour of' becomes 'probable' (I can't justify this - if you can you should give a lot more credence to this answer).
'im'+'probable'='IMPROBABLE'
'half-hearted waffle?' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for improbable that I've seen before include "Far-fetched" , "Not likely" , "Highly unlikely" .)