Bond's boss played by Dench holds key - not one to underestimate! (8)
I believe the answer is:
misjudge
'underestimate' is the definition.
(thesaurus)
'bond's boss played by dench holds key not one' is the wordplay.
I cannot quite understand how this works, but
'key' could be 'g' (I've seen this in other clues) and 'g' is located in the answer.
'one' could be 'i' (Roman numeral) and 'i' is present in the answer.
'bond's boss' could be 'm' (I've seen this in other clues) and 'm' is found in the answer.
This explanation may well be incorrect...
'to' acts as a link.
(Other definitions for misjudge that I've seen before include "Wrongly assess" , "Form wrong opinion of, estimate incorrectly" , "Get wrong" , "Estimate wrongly" , "Wrongly reckon" .)