Follow river into dock (5)
I believe the answer is:
trail
'follow' is the definition.
(trail can mean to follow in secret)
'river into dock' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'into' indicates putting letters inside.
'dock' becomes 'tail' (synonyms).
'r' placed into 'tail' is 'TRAIL'.
(Other definitions for trail that I've seen before include "Beaten path; be in losing position" , "Pursue" , "Country path; follow" , "Beaten path; track" , "Get behind" .)