Famous boxer had to work hand in glove (9)
'famous boxer' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'had to work hand in glove' is the wordplay.
'had' becomes 'ate' (e.g. I had an ice cream).
'to' means one lot of letters go next to another (I've seen this in other clues).
'work hand in glove' becomes 'cooper' (I can't explain this - if you can you should give a lot more credence to this answer).
'ate' put after 'cooper' is 'CO-OPERATE'.
Can you help me to learn more?
(Another definition for co-operate that I've seen is " Work together".)
(Other definitions for cooperate that I've seen before include "Work together on a common enterprise" , "Help" , "getting act together" , "Work to the same end" , "Play ball with" .)