Twain's Huck (4)
I believe the answer is:
'twain's huck' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for finn that I've seen before include "Huckleberry - -" , "Trollope's Phineas; one from Tampere" , "eg Sibelius" , "Nordic" , "Twain's Huck; Trollope's Phineas" .)