Son in undertaking (8)
I believe the answer is:
swearing
'undertaking' is the definition.
The answer and definition can be both to do with communicating as well as being present participle verbs.
Maybe they are linked in a way I don't understand?
'son in' is the wordplay.
I cannot really understand how this works, but
'son' could be 's' (genealogical abbreviation for son) and 's' is found within the answer.
'in' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for swearing that I've seen before include "blue emissions?" , "promising" , "Using bad language" , "Vowing" , "Expressing annoyance" .)