A President, terribly dull (10)
I believe the answer is:
pedestrian
'dull' is the definition.
('pedestrian' can be similar in meaning to 'dull')
'a president terribly' is the wordplay.
'terribly' indicates anagramming the letters.
'a'+'president'='apresident'
'apresident' anagrammed gives 'PEDESTRIAN'.
(Other definitions for pedestrian that I've seen before include "A person travelling on foot" , "he goes for a walk" , "nothing much to write home about" , "not very fast" , "footman" .)