Yelling to mum on an excursion (8)
I believe the answer is:
shouting
'yelling' is the definition.
(I know that yelling can be written as shouting)
'mum on an excursion' is the wordplay.
'mum' becomes 'sh' (both are used to tell someone to be quiet).
'on' means one lot of letters go next to another.
'an excursion' becomes 'outing' ('outing' can be a synonym of 'excursion').
'sh'+'outing'='SHOUTING'
'to' is the link.
(Other definitions for shouting that I've seen before include "Talking very loudly" , "Talked very loudly" .)