Strike a bit of an idiot as fast (5)
I believe the answer is:
rapid
'fast' is the definition.
(similar in meaning)
'strike a bit of an idiot' is the wordplay.
'strike' becomes 'rap' (synonyms).
'a bit of an idiot' becomes 'id' (I can't explain this - if you can you should give a lot more credence to this answer).
'rap'+'id'='RAPID'
'as' acts as a link.
Can you help me to learn more?
(Other definitions for rapid that I've seen before include "Happening quickly" , "Speedy (5)" , "Quickly accomplished" , "Fast, speedy" , "Brisk" .)