Prez on a fiver (3)
I believe the answer is:
abe
'prez on a fiver' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for abe that I've seen before include "Eg, president Lincoln" , "President Lincoln was known as Honest ..." , "Lincoln, familiarly" , "Lincoln's diminutive" .)