Crook? Boycott pointless diet (6)
I believe the answer is:
bandit
'crook?' is the definition.
(I know that bandit is a type of thief)
'boycott pointless diet' is the wordplay.
'boycott' becomes 'ban' (I've seen this in another clue).
'pointless diet' becomes 'dit' (I can't justify this - if you can you should give a lot more credence to this answer).
'ban'+'dit'='BANDIT'
Can you help me to learn more?
(Other definitions for bandit that I've seen before include "Armed robber in the Wild West" , "Member of a robber gang" , "Enemy aircraft" , "Highwayman" , "Dan bit the robber badly" .)