Light that warns companion leaving compartment (5)
I believe the answer is:
amber
'light that warns' is the definition.
(traffic lights)
'companion leaving compartment' is the wordplay.
'companion' becomes 'ch' (I've seen this in other clues).
'leaving' indicates named letters should be taken away.
'compartment' becomes 'chamber' (I've seen this before).
'chamber' with 'ch' taken away is 'AMBER'.
(Other definitions for amber that I've seen before include "A yellowy resin" , "Colour of warning" , "Girl" , "warning signal" , "Neither red nor green?" .)