One’s taken wrong line in the dark, lacking this? (4,5)
I believe the answer is:
neon light
'lacking this?' is the definition.
I know nothing about this answer so I can't tell whether this works.
'one's taken wrong line in the dark' is the wordplay.
'taken wrong' indicates an anagram.
'line' becomes 'l'.
'in' is an insertion indicator.
'the dark' becomes 'night' (night is a kind of dark).
'one' with letters rearranged gives 'eon'.
'eon'+'l'='eonl'
'eonl' going within 'night' is 'NEON LIGHT'.
(Other definitions for neon light that I've seen before include "Type of illumination" , "Source of illumination" , "What may shine" , "Advertising sign, perhaps" , "Illumination used for advertising" .)