A different part of flower storing oxygen
I believe the answer is:
another
'a different' is the definition.
(I've seen this before)
'part of flower storing oxygen' is the wordplay.
'part of flower' becomes 'anther' ('anther' is part of 'flower').
'storing' means one lot of letters goes inside another.
'oxygen' becomes 'O' (atomic symbol for oxygen).
'anther' placed around 'o' is 'ANOTHER'.
(Other definitions for another that I've seen before include "encore?" , "A different one" , "Additional, one more" , "A further (item)" , "A different (thing)" .)