Vowed southwest will get valuable mineral (5)
I believe the answer is:
swore
'vowed' is the definition.
(swearing is a kind of vowing)
'southwest will get valuable mineral' is the wordplay.
'southwest' becomes 'sw' ('SW' can be a synonym of 'southwest').
'will get' says to put letters next to each other.
'valuable mineral' becomes 'ore' (I've seen this before).
'sw'+'ore'='SWORE'
(Other definitions for swore that I've seen before include "used bad language" , "Took oath or cursed" , "Uttered profanities or avowed" , "Blasphemed" .)