Mess about with unknown earthenware (7)
I believe the answer is:
pottery
'earthenware' is the definition.
(I know that earthenware can be written as pottery)
'mess about with unknown' is the wordplay.
'mess about' becomes 'potter' (I've seen this before).
'with' means one lot of letters go next to another.
'unknown' becomes 'y' (algebra).
'potter'+'y'='POTTERY'
(Other definitions for pottery that I've seen before include "China, for example" , "Ceramic ware" , "Clayware" , "Items of fired clay" , "baked goods" .)