Letters from Oscar certainly seem few and far between (6)
I believe the answer is:
scarce
'few and far between' is the definition.
(I've seen this before)
'letters from oscar certainly seem' is the wordplay.
'letters from' says the answer is hidden in the clue (a selection of letters from the clue).
'SCARCE' is hidden within 'oscar certainly seem'.
(Other definitions for scarce that I've seen before include "Not plentiful, hard to find" , "Scant, meagre" , "Not often come across" , "Rarely found" , "Uncommon" .)