Oat chips spread over single nut (9)
I believe the answer is:
pistachio
'nut' is the definition.
(pistachio is a kind of nut)
'oat chips spread over single' is the wordplay.
'spread' is an anagram indicator.
'over' means one lot of letters goes inside another.
'single' becomes 'i' (one).
'oat'+'chips'='oatchips'
'oatchips' is an anagram of 'pstachio'.
'pstachio' placed around 'i' is 'PISTACHIO'.
(Other definitions for pistachio that I've seen before include "light green" , "Green nut" , "Type of nut" , "nut-bearing tree" , "Nut - it is a chop (anag)" .)