Clue: Everyman affectedly starts to speechify in extremely shapeless garment (6)
Answer: ONESIE
Explanation: Definition: shapeless garment
A charade of ONE ("Everyman affectedly' – high falutin' use of the impersonal personal pronoun) plus SIE ('starts to **S**peechify** I**n **E**xtremely'). Note that here 'extremely' does not indicate first and last letters of something; a little insider joke.