One recently arrived dined under gaslight (7)
I believe the answer is:
neonate
'one recently arrived' is the definition.
(term for a newborn child or animal)
'dined under gaslight' is the wordplay.
'dined' becomes 'ate'.
'under' means one lot of letters go next to another.
'gaslight' becomes 'neon' (light using neon gas).
'ate' put after 'neon' is 'NEONATE'.
(Other definitions for neonate that I've seen before include "recently delivered" , "New baby" , "Newborn baby" , "produced baby" , "Recently born child" .)