Becomes hard to follow case of European political theorist (6)
I believe the answer is:
engels
'political theorist' is the definition.
(philosopher Friedrich Engels)
'becomes hard to follow case of european' is the wordplay.
'becomes hard' becomes 'gels' (gel can mean to set or harden).
'to follow' means one lot of letters go next to another.
'case of' means to remove the middle letters (outsides of).
'european' with its middle removed is 'en'.
'gels' put after 'en' is 'ENGELS'.
(Other definitions for engels that I've seen before include "Early socialist" , "Co-author of Karl Marx" , "Pioneer of communism" , "German political philosopher who collaborated with Marx" , "Friedrich --, associate of Karl Marx" .)