Take on oxygen in publicity exercise (5)
I believe the answer is:
adopt
'take on' is the definition.
('adopt' can be a synonym of 'take on')
'oxygen in publicity exercise' is the wordplay.
'oxygen' becomes 'o' (atomic symbol for oxygen).
'in' indicates putting letters inside.
'publicity' becomes 'ad' (advert).
'exercise' becomes 'pt'.
'ad'+'pt'='adpt'
'o' inserted inside 'adpt' is 'ADOPT'.
(Other definitions for adopt that I've seen before include "Espouse" , "Take on (another's child)" , "Take responsibility for" , "Embrace" , "make one's own" .)