Man was brave as exterior dimmed (8)
I believe the answer is:
darkened
'dimmed' is the definition.
('darken' can be a synonym of 'dim')
'man was brave as exterior' is the wordplay.
'man' becomes 'Ken' (short for Kenneth).
'was brave as' becomes 'dared' (I am not sure about this - if you are sure you should believe this answer much more).
'exterior' means one lot of letters goes inside another (some letters go on the outside of others).
'ken' placed into 'dared' is 'DARKENED'.
Can you help me to learn more?
(Other definitions for darkened that I've seen before include "lost the sun" , "Made less light" , "obscured" , "Made or became less bright or cheerful" , "Delighted possibly" .)