Speak of charge applied to oxygen (5)
I believe the answer is:
orate
'speak' is the definition.
(orating is a kind of speaking)
'charge applied to oxygen' is the wordplay.
'charge' becomes 'rate' (rate is a kind of charge).
'applied to' says to put letters next to each other.
'oxygen' becomes 'O' (atomic symbol for oxygen).
'rate' put after 'o' is 'ORATE'.
'of' is the link.
(Other definitions for orate that I've seen before include "Harangue" , "Make a lengthy speech" , "Speechify" , "Speak publicly" , "Deliver a speech" .)