When filmmakers produce big mainstream films that sneak in a radical political message, this is entryist filmmaking. Examples noted by Mark Kermode - an Old Trot himself - have included Serenity and 28 Weeks Later.
Simon Mayo refused to believe the word "entryist" existed, and was backed up when a big search for it on Dictionary.com revealed nothing.
When Simon asked Mark what his younger, Communist self would have made of him running around the quad at Eton, Mark replied that he would have assumed it was entryist insurgency mission to destroy capitalism from within. Either that, or he'd just sold out.