Call out “Mum! There’s a child outside” (6)
I believe the answer is:
summon
'call' is the definition.
(I know that call is a is a more specific form of the action of summon)
'out mum there's a child outside' is the wordplay.
'out' is an anagram indicator.
'there's a child' becomes 'son' (male child).
'outside' is an insertion indicator (some letters go outside others).
'mum' anagrammed gives 'umm'.
'umm' inserted within 'son' is 'SUMMON'.
(Other definitions for summon that I've seen before include "Order to attend" , "Call to attend on an official matter" , "Call (people) to attend" , "Convene" , "Command to come - to court?" .)