Betray in the manner of the deserter aboard (3,2)
I believe the answer is:
rat on
'betray' is the definition.
(synonyms)
'deserter aboard' is the wordplay.
'deserter' becomes 'rat' (both can mean a traitor).
'aboard' becomes 'on' (eg on a ship or train).
'rat'+'on'='RAT ON'
'in the manner of the' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for rat on that I've seen before include "desert" , "grass up" , "shop" , "Betray" , "Give away" .)