Devon and Cornwall listener's vow (5)
I believe the answer is:
swear
'vow' is the definition.
(I know that swear is a type of vow)
'devon and cornwall listener's' is the wordplay.
'devon and cornwall' becomes 'SW' (Cornwall and Devon are in the south west England).
'listener' becomes 'ear' (body part which hears or listens).
'sw'+'ear'='SWEAR'
(Other definitions for swear that I've seen before include "Use bad language" , "Affirm" , "Use indecent or blasphemous language" , "not to mince one's words" , "Express annoyance" .)