One pound off reasonably unsubstantial Christmas decoration (5,5)
I believe the answer is:
fairy light
'christmas decoration' is the definition.
(fairy lights may be used as Christmas decorations)
'one pound off reasonably unsubstantial' is the wordplay.
'one pound' becomes 'L' (the pound sign is based on the letter L).
'off' indicates named letters should be taken away.
'reasonably' becomes 'fairly' (synonyms).
'unsubstantial' becomes 'light'.
'fairly' with 'l' taken out is 'fairy'.
'fairy'+'light'='FAIRY LIGHT'
(Other definitions for fairy light that I've seen before include "one lit on tree perhaps" , "Decorative twinkler" .)