Digit so extreme, ultimately? (3)
I believe the answer is:
toe
'digit' is the definition.
(toe is a kind of digit)
'so extreme ultimately?' is the wordplay.
'so' becomes 'to' (I can't justify this - if you can you should give a lot more credence to this answer).
'ultimately?' suggests the final letters (I've seen 'ultimately' mean this).
The last letter of 'extreme' is 'e'.
'to'+'e'='TOE'
(Other definitions for toe that I've seen before include "Small member" , "one digit?" , "Pedal extremity" , "One's nailed" , "Digit; kick" .)