Develop simple diet — you might be punished for breaking it! (5,5)
I believe the answer is:
speed limit
'you might be punished for breaking it' is the definition.
(drivers who break speed limits might be punished)
'develop simple diet' is the wordplay.
'develop' is an anagram indicator.
'simple'+'diet'='simplediet'
'simplediet' is an anagram of 'SPEED-LIMIT'.
(Other definitions for speed limit that I've seen before include "maximum rate by law" , "Legal cap on drivers" , "Fine results maybe after exceeding this" , "Rule facing drivers" , "seventy, at most" .)