Track dog crossing river (5)
I believe the answer is:
trail
'track' is the definition.
(synonyms)
'dog crossing river' is the wordplay.
'dog' becomes 'tail' (synonyms).
'crossing' is an insertion indicator.
'river' becomes 'r' (abbreviation for river).
'tail' enclosing 'r' is 'TRAIL'.
(Other definitions for trail that I've seen before include "Pursue" , "Scent or track followed in hunting" , "Beaten path through the countryside" , "Track, route" , "Rough path" .)