Every now and then, tunnel vista is dark (5)
I believe the answer is:
unlit
'is dark' is the definition.
(similar in meaning)
'every now and then tunnel vista' is the wordplay.
'every now and then' means one should take alternating letters (letters selected at regular intervals).
'tunnel'+'vista'='tunnelvista'
The alternating letters of 'tunnelvista' are 'UNLIT'.
(Other definitions for unlit that I've seen before include "Dark, gloomy" , "Lacking illumination" , "not switched on" , "Perhaps invisible" , "Shadowy" .)