Puts a hex on first jack in last three boxes (6)
I believe the answer is:
jinxes
'puts a hex on' is the definition.
(I've seen this in another clue)
'jack in last three boxes' is the wordplay.
'jack' becomes 'J' (playing card abbreviation).
'last three boxes' becomes 'xes' (I can't explain this - if you can you should believe this answer much more).
'j'+'in'+'xes'='JINXES'
'first' is the link.
Can you help me to learn more?
(Other definitions for jinxes that I've seen before include "Things or people that bring bad luck" , "Evil spells" .)