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 really understand 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 see.
This explanation may well be incorrect...
'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" .)