Follow, follow around river (5)
I believe the answer is:
trail
'follow' is the definition.
(thesaurus)
'follow around river' is the wordplay.
'follow' becomes 'tail' (I've seen this before).
'around' means one lot of letters goes inside another.
'river' becomes 'r'.
'tail' enclosing 'r' is 'TRAIL'.
(Other definitions for trail that I've seen before include "Stalk" , "Follow - track" , "Track or scent used in following someone" , "Forest path" , "Mark left where something has passed - follow it" .)