New baby pinched by two fiends in the night (6)
I believe the answer is:
incubi
'fiends in the night' is the definition.
(plural of incubus, a type of demon)
'new baby pinched by two' is the wordplay.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'baby' becomes 'cub' (cub can mean a baby).
'pinched by' means one lot of letters goes inside another (inserted letters get pinched or taken).
'two' becomes 'II' (Roman numerals).
'n'+'cub'='ncub'
'ncub' inserted within 'ii' is 'INCUBI'.
(Other definitions for incubi that I've seen before include "nightmares" , "Demons of the night" .)