Joker on table, showing boxing material (9)
I believe the answer is:
cardboard
'boxing material' is the definition.
'cardboard' can be an answer for 'material' (cardboard is a kind of material). I'm unsure of the 'boxing' bit.
'joker on table' is the wordplay.
'joker' becomes 'card' (joker is a type of playing card).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'table' becomes 'board' (board is a kind of table).
'card'+'board'='CARDBOARD'
'showing' is the link.
Can you help me to learn more?
(Other definitions for cardboard that I've seen before include "A stiff thick paper" , "stiff material" , "without substance" .)