Regularly cold and stormy outside — most unlikely to see the sun? (9)

'most unlikely to see the sun?' is the definition.
(most cloudy)

'regularly cold and stormy outside' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'and' says to put letters next to each other.
'stormy' is an anagram indicator (stormy suggests disruption).
The alternate letters of 'cold' are 'cl'.
'outside' with letters rearranged gives 'oudiest'.
'cl'+'oudiest'='CLOUDIEST'