Unusual tea leaving baby with gas (4)
I believe the answer is:
neon
'gas' is the definition.
(I've seen this before)
I cannot understand how the remainder of the clue works.
(Other definitions for neon that I've seen before include "Colourless gas, used in lighting" , "Brightly coloured advertising sign" , "Gas used in fluorescent lighting" , "Colourless, odourless rare gas" , "illuminating agent" .)