One more part of flower absorbing oxygen (7)
I believe the answer is:
another
'one more' is the definition.
(I've seen this before)
'part of flower absorbing oxygen' is the wordplay.
'part of flower' becomes 'anther' ('anther' is part of 'flower').
'absorbing' means one lot of letters goes inside another.
'oxygen' becomes 'O' (atomic symbol for oxygen).
'anther' going around 'o' is 'ANOTHER'.
(Other definitions for another that I've seen before include "Player or person not yet named or selected" , "Somebody else" , "Additional, one more" , "encore?" , "A different (thing)" .)