Lag, right in the back (5)
I believe the answer is:
trail
'lag' is the definition.
(trailing is a kind of lagging)
'right in the back' is the wordplay.
'right' becomes 'r' (common abbreviation).
'in the' is an insertion indicator.
'back' becomes 'tail' (tail is a kind of back).
'r' put within 'tail' is 'TRAIL'.
(Other definitions for trail that I've seen before include "Scent or track followed in hunting" , "Track, route" , "Straggle" , "Path or track through wild or hilly country" , "Way" .)