Mark is terribly verbose (7)
I believe the answer is:
observe
'mark' is the definition.
(I've seen this before)
'terribly verbose' is the wordplay.
'terribly' indicates anagramming the letters.
'verbose' is an anagram of '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.)" .)