Followed across river, being towed (7)
I believe the answer is:
trailed
'towed' is the definition.
(trailing is a kind of towing)
'followed across river being' is the wordplay.
'followed' becomes 'tailed' (I've seen this before).
'across' indicates putting letters inside.
'river being' becomes 'r' (I am not sure about this - if you are sure you should believe this answer much more).
'tailed' enclosing 'r' is 'TRAILED'.
Can you help me to learn more?
(Other definitions for trailed that I've seen before include "Lagged behind" , "Dragged" , "Followed or tracked after" .)