Follow only half of Marx’s doctrine (5)
I believe the answer is:
dogma
'doctrine' is the definition.
(dogma is a kind of doctrine)
'follow only half of marx's' is the wordplay.
'follow' becomes 'dog' (I've seen this before).
'only half of' indicates taking half.
'marx' with half the letters taken is 'ma'.
'dog'+'ma'='DOGMA'
(Other definitions for dogma that I've seen before include "Blind faith required" , "Fixed belief" , "Code of beliefs accepted as authoritative" , "Firm doctrine" , "Religious doctrine proclaimed as true without proof" .)