Runs inside dog track (5)
I believe the answer is:
trail
'track' is the definition.
(synonyms)
'runs inside dog' is the wordplay.
'runs' becomes 'r' (cricket abbreviation).
'inside' means one lot of letters goes inside another.
'dog' becomes 'tail' (synonyms).
'r' placed within 'tail' is 'TRAIL'.
(Other definitions for trail that I've seen before include "Forest path" , "Drag; path" , "Beaten path; follow" , "Track - be behind" , "Rough path" .)