A mere idealist forced to disappear (13)
I believe the answer is:
dematerialise
'disappear' is the definition.
('disappear'->'dematerialize' is in my internal thesaurus and 'ize'->'ise')
'a mere idealist forced' is the wordplay.
'forced' indicates an anagram.
'a'+'mere'+'idealist'='amereidealist'
'amereidealist' is an anagram of 'DEMATERIALISE'.
'to' is the link.
(Other definitions for dematerialise that I've seen before include "Lose physical presence" , "Cease to have physical presence" , "Vanish" .)