Gas is volatile, and countries have meetings (12)
I believe the answer is:
assignations
'meetings' is the definition.
(assignation is a kind of meeting)
'gas is volatile and countries' is the wordplay.
'volatile' indicates anagramming the letters.
'and' means one lot of letters go next to another.
'countries' becomes 'nations' ('nation' can be a synonym of 'country'**).
'gas'+'is'='gasis'
'gasis' with letters rearranged gives 'assig'.
'assig'+'nations'='ASSIGNATIONS'
'have' is the link.
(Another definition for assignations that I've seen is " Lovers' meetings".)