Being unappreciative about holiday illumination (5,8)
I believe the answer is:
strip lighting
'illumination' is the definition.
(I've seen this before)
'being unappreciative about holiday' is the wordplay.
'being unappreciative' becomes 'slighting' (to slight someone is to snub them).
'about' means one lot of letters goes inside another.
'holiday' becomes 'trip' (I've seen this before).
'slighting' placed around 'trip' is 'STRIP LIGHTING'.
(Other definitions for strip lighting that I've seen before include "overhead feature" , "Fluorescent tubes" , "which proves illuminating" , "illumination" , "It provides visibility" .)