Poorly-lit weaving machine blocking outside of gallery (6)
I believe the answer is:
gloomy
'poorly-lit' is the definition.
(gloomy means dark or dim)
'weaving machine blocking outside of gallery' is the wordplay.
'weaving machine' becomes 'loom' (machine for weaving thread).
'blocking' means one lot of letters goes inside another.
'outside of' suggests removing the centre.
'gallery' with its centre taken out is 'gy'.
'loom' placed within 'gy' is 'GLOOMY'.
(Other definitions for gloomy that I've seen before include "Downcast" , "Dismal - despondent" , "Dim - pessimistic" , "'Morose, glum (6)'" , "Dark - pessimistic" .)