What is the right single word for an intellectual (often a journalist) who is not a member of a political party, but justifies the party's causes to the general public and secretly takes benefit from them in return?
A shill is a tout or promoter, with the connotation of figure who endorses a product or service— or candidate or party— on a supposedly independent basis, when in fact he or she is in the service of the person or organization benefiting from the endorsement.
A related phrase is fellow traveller,“One who sympathizes with the aims or beliefs of an organization, without belonging to it; most often applied to a Communist sympathizer”. Technically, the phrase applies fairly well in the current context, except that people are unlikely to automatically suppose a fellow traveller is an intellectual or a journalist or secretly receives benefits.
partisan (noun) an adherent or supporter of a person, group, party, or cause, especially a person who shows a biased, emotional allegiance. (Dictionary.com)
Two words not previously suggested are hack and hireling. According to R. L. Chapman and B.A. Kipfer, Dictionary of American Slang, Third Edition (1995):
hack 8 n (also ) by 1810 A professional, usu freelance, writer who works to order[.] This sense belongs to hack [definition 1, meaning "A taxicab"], reflecting the notion that such a writer was for hire like a horse, ...
Merriam-Webster's Eleventh Collegiate Dictionary (2003) offers this definition:
hack n. [short for *hack*ney] 3 a : a person who works solely for mercenary reasons : hireling
As for hireling, the Eleventh Collegiate says this:
hireling n : a person who serves for hire esp. for purely mercenary motives
Other options that might suit the situation are apologist, mouthpiece, mercenary, and hired gun.