Go too far, finding Dover terribly old (6)
I believe the answer is:
overdo
'go too far' is the definition.
(I've seen this before)
'dover terribly old' is the wordplay.
'terribly' indicates an anagram.
'old' becomes 'o' (abbreviation).
'dover' with letters rearranged gives 'overd'.
'overd'+'o'='OVERDO'
'finding' acts as a link.
(Other definitions for overdo that I've seen before include "Cook too long or act excessively" , "Cook wrongly" , "Take too far" , "Be excessive, say cook too long" , "Work too much" .)