Mad curve you start (5)
I believe the answer is:
loopy
'mad' is the definition.
(I've seen this before)
'curve you start' is the wordplay.
I cannot quite understand how this works, but
'you start' could be 'y' (I've seen this in other clues) and 'y' is found in the answer.
The remaining letters 'loop' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for loopy that I've seen before include "Mad" , "out to lunch?" , "Round the bend" , "Slightly crazy" , "twisted?" .)