Follower of slithy thing from the East in river (7)
I believe the answer is:
devotee
'follower' is the definition.
(devotee is a kind of follower)
'slithy thing from the east in river' is the wordplay.
'slithy thing' becomes 'tove' ('slithy toves' in Lewis Carroll's 'Jabberwocky').
'from the east' says the letters should be written in reverse.
'in' indicates putting letters inside.
'river' becomes 'dee' (River Dee).
'tove' back-to-front is 'evot'.
'evot' going within 'dee' is 'DEVOTEE'.
'of' is the link.
(Other definitions for devotee that I've seen before include "Big fan" , "Person who is utterly loyal and committed to someone" , "Ardent follower and admirer" , "Fan, enthusiast" , "dedicated person" .)