Deserter turns to pitch (3)
I believe the answer is:
tar
'pitch' is the definition.
(synonyms)
'deserter turns' is the wordplay.
'deserter' becomes 'rat' (both can mean a traitor).
'turns' shows that the letters should be reversed in order.
'rat' in reverse letter order is 'TAR'.
'to' acts as a link.
(Other definitions for tar that I've seen before include "Black coat" , "Sailor and dressing for roads" , "Seaman; pitch" , "Product of burning tobacco" , "one may need to pay on deck" .)