Some rebels have fallen in with a Soviet leader (5)
I believe the answer is:
lenin
'a soviet leader' is the definition.
(I've seen this before)
'some rebels have fallen in' is the wordplay.
'some rebels' becomes 'len' (I am not sure about this - if you are sure you should believe this answer much more).
'have' means one lot of letters goes inside another.
'fallen' indicates anagramming the letters.
'in' is an anagram of 'ni'.
'len' enclosing 'ni' is 'LENIN'.
'with' acts as a link.
Can you help me to learn more?
(Other definitions for lenin that I've seen before include "Revolutionary leader" , "Communist leader" , "Soviet Union's first premier" , "Russian revolutionary" , "Marxist theoretician" .)