Sluggish type’s second dejected trainer (9)
I believe the answer is:
slowcoach
'sluggish type's' is the definition.
(someone who moves or acts slowly)
'second dejected trainer' is the wordplay.
'second' becomes 's' (common abbreviation).
'dejected' becomes 'low' (similar in meaning).
'trainer' becomes 'coach' (coach is a kind of trainer).
's'+'low'+'coach'='SLOWCOACH'
(Other definitions for slowcoach that I've seen before include "someone sluggish" , "plodder" , "Laggard" , "Tortoise-like walker" , "One who takes time to do things" .)