Dress-wearing friend of Dorothy is to behave extravagantly (2,2,4)
I believe the answer is:
go to town
'to behave extravagantly' is the definition.
(I've seen this before)
'dress-wearing friend of dorothy' is the wordplay.
'dress' becomes 'gown' (gowning is a kind of dressing).
'wearing' indicates putting letters inside.
'friend of dorothy' becomes 'toto' (Dorothy's pet dog in The Wizard of Oz).
'gown' placed around 'toto' is 'GO TO TOWN'.
'is' is the link.
(Other definitions for go to town that I've seen before include "Behave without restraint" , "Do something thoroughly" , "Be most thorough" , "Act without restraint" , "To do well" .)