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' is the wordplay.
I cannot quite see how this works, but
'to' is within the answer.
The remaining letters 'ran' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'will' is the link.
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" .)