Gas worried baby (7)
I believe the answer is:
neonate
'baby' is the definition.
(medical term for a newborn child)
'gas worried' is the wordplay.
'gas' becomes 'neon' (neon is a gas).
'worried' becomes 'ate' (something worrying may eat someone up).
'neon'+'ate'='NEONATE'
(Other definitions for neonate that I've seen before include "Young infant" , "very young child" , "New baby" , "One in delivery ward" , "Young baby" .)