In the tube a constant light (6)
I believe the answer is:
beacon
'light' is the definition.
(I've seen this before)
'in the tube a constant' is the wordplay.
'in' says the answer is hidden in the clue.
'BEACON' is hidden in the letters of 'tube a constant'.
(Other definitions for beacon that I've seen before include "Prominent light on a hill" , "Warning fire or signal" , "Guiding light?" , "Raised signal fire or light" , "Watch fire" .)