Despair as novel about Portugal vanishes (10)
I believe the answer is:
disappears
'vanishes' is the definition.
('disappear' can be a synonym of 'vanish')
'despair as novel about portugal' is the wordplay.
'novel' is an anagram indicator.
'about' indicates putting letters inside.
'portugal' becomes 'p' (common abbreviation eg on car number plates).
'despair'+'as'='despairas'
'despairas' with letters rearranged gives 'disapears'.
'disapears' enclosing 'p' is 'DISAPPEARS'.
(Other definitions for disappears that I've seen before include "becomes lost" , "goes missing" .)