One more part of flower absorbing oxygen (7)
I believe the answer is:
another
'one more' is the definition.
(another means one more)
'part of flower absorbing oxygen' is the wordplay.
'part of flower' becomes 'anther' ('anther' is part of 'flower').
'absorbing' indicates putting letters inside.
'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)" .)