Christmas decoration can seem extremely large initially (6)
I believe the answer is:
tinsel
'christmas decoration' is the definition.
(tinsel is strewn around at Christmas)
'can seem extremely large initially' is the wordplay.
'can' becomes 'tin' (synonyms).
'initially' says to take the initial letters.
The initial letters of 'seem extremely large' is 'sel'.
'tin'+'sel'='TINSEL'
(Other definitions for tinsel that I've seen before include "decorating material" , "Gaudy decoration" , "Cheap shiny stuff" , "Strip of glittering paper" , "Christmas foil" .)