A sympathiser with the group, Rev Low, fell later - sort of! (6,9)
I believe the answer is:
fellow traveller
'a sympathiser' is the definition.
(I've seen this before)
'rev low fell later sort of' is the wordplay.
'sort of' indicates an anagram.
'rev'+'low'+'fell'+'later'='revlowfelllater'
'revlowfelllater' anagrammed gives 'FELLOW-TRAVELLER'.
'with the group' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for fellow traveller that I've seen before include "possibly 18 [PARTISAN]" , "Like monk for nun in Chaucer, for example" , "Communist sympathiser" , "left supporter" , "one with leftist leanings" .)