Lag behind dog crossing river (5)
I believe the answer is:
trail
'lag behind' is the definition.
(I've seen this before)
'dog crossing river' is the wordplay.
'dog' becomes 'tail' (synonyms).
'crossing' indicates putting letters inside (to cross can mean to go over or around).
'river' becomes 'r' (abbreviation for river).
'tail' placed around 'r' is 'TRAIL'.
(Other definitions for trail that I've seen before include "advertise upcoming programmes" , "Rough path" , "Forest path" , "Spoor - lag behind" , "Mark left where something has passed - follow it" .)