Mark is terribly verbose (7)
I believe the answer is:
observe
'mark' is the definition.
(I know that mark is a more specific form of the action observe)
'terribly verbose' is the wordplay.
'terribly' indicates an anagram.
'verbose' with letters rearranged gives 'OBSERVE'.
'is' acts as a link.
(Other definitions for observe that I've seen before include "Keep; notice" , "Notice; study" , "Watch carefully" , "Keep an eye on" , "Obverse (anag.)" .)