Nonsense about unknown work being source of material for play (7)
I believe the answer is:
toyshop
'source of material for play' is the definition.
(shop selling materials to play with)
'nonsense about unknown work' is the wordplay.
'nonsense' becomes 'tosh' (tosh is a kind of nonsense).
'about' is an insertion indicator.
'unknown' becomes 'y' (algebra).
'work' becomes 'op' (common abbreviation for 'opus').
'tosh' enclosing 'y' is 'toysh'.
'toysh'+'op'='TOYSHOP'
'being' is the link.
(Other definitions for toyshop that I've seen before include "Source of amusements" , "Seller of games, dolls, etc" , "Where to buy playthings" , "Hamleys, say," , "store for kids" .)