Diana and Reginald redesigned the crest (5)
I believe the answer is:
ridge
'the crest' is the definition.
(geographical term)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for ridge that I've seen before include "roof-tree" , "Long raised band" , "Line of higher ground" , "Long mountain crest or roof life" , "Wrinkle" .)