In the tube a constant light is seen (6)
I believe the answer is:
beacon
'light is seen' is the definition.
I can't tell whether this defines the answer.
'in the tube a constant' is the wordplay.
'in' indicates a hidden word.
'BEACON' can be found hidden inside 'tube a constant'.
Can you help me to learn more?
(Other definitions for beacon that I've seen before include "Fire on a hill, lit as a signal" , "Warning light on hill or cliff" , "Watch fire" , "Shiner" , "Raised signal fire or light" .)