Allowed to remove cap that's dreadful (5)
I believe the answer is:
awful
'dreadful' is the definition.
(synonyms)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for awful that I've seen before include "Disgusting" , "Appalling, terrible" , "Dreadful, really bad" , "Really terrible, ghastly" , "Exceptionally bad" .)