Part of plant, one more deficient in oxygen (6)
I believe the answer is:
anther
'part of plant' is the definition.
(I've seen this before)
'one more deficient in oxygen' is the wordplay.
'one more' becomes 'another' (I've seen this before).
'deficient in' indicates named letters should be taken away.
'oxygen' becomes 'O' (atomic symbol for oxygen).
'another' with 'o' taken out is 'ANTHER'.
(Other definitions for anther that I've seen before include "Pollen-producing part of a flower" , "Pollen part of flower" , "Pollen-holding part of stamen" , "(Actors) leave stage" , "Ran the pollen into it" .)