Limit short show with clowns, medium and writer (12)
I believe the answer is:
circumscribe
'limit' is the definition.
(synonyms)
'short show with clowns medium and writer' is the wordplay.
'short' means to remove the last letter.
'show with clowns' becomes 'circus'.
'medium' becomes 'm' (as in UK clothing sizes).
'and' means one lot of letters go next to another.
'writer' becomes 'scribe' (scribe is a kind of writer).
'circus' with its last letter taken away is 'circu'.
'circu'+'m'+'scribe'='CIRCUMSCRIBE'
(Other definitions for circumscribe that I've seen before include "Restrict; draw round" , "Restrict, limit" , "put in enclosed space" , "Confine within limited range" .)