To get ahead, the dirty fellow will betray! (3,2)
I believe the answer is:
rat on
'betray' is the definition.
(synonyms)
'to get ahead the dirty fellow will' is the wordplay.
I cannot really understand how this works, but
'to' is found in the answer.
The remaining letters 'ran' is a valid word which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for rat on that I've seen before include "grass up" , "Betray" , "shop" , "Give away" , "desert" .)