Small sad trainer with laggard (9)
I believe the answer is:
slowcoach
'laggard' is the definition.
(I know that slowcoach is a type of laggard)
'small sad trainer' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'sad' becomes 'low' (I've seen this in other clues).
'trainer' becomes 'coach' (coach is a kind of trainer).
's'+'low'+'coach'='SLOWCOACH'
'with' is the link.
(Other definitions for slowcoach that I've seen before include "One who takes time to do things" , "One who lacks liveliness, like old vehicle" , "someone sluggish" , "Sluggish person like tardy trainer" , "One who's not very fast" .)