Frivolous boxer (11)
I believe the answer is:
lightweight
'frivolous boxer' is the definition.
'lightweight' can be an answer for 'boxer' (lightweight is a kind of boxer). I am not sure about the 'frivolous' bit.
'frivolous boxer' is the wordplay.
'frivolous' becomes 'light' (light can mean trivial or unimportant).
'boxer' becomes 'weight' (I can't explain this - if you can you should give a lot more credence to this answer).
'light'+'weight'='LIGHTWEIGHT'
Can you help me to learn more?
(Other definitions for lightweight that I've seen before include "Fighter" , "Of little substance" , "Insubstantial" , "Carrying little punch?" , "of little importance" .)