Dawdler, small, with miserable trainer (9)
I believe the answer is:
slowcoach
'dawdler' is the definition.
(I know that slowcoach is a type of dawdler)
'small with miserable trainer' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'with' says to put letters next to each other.
'miserable' becomes 'low' (synonyms).
'trainer' becomes 'coach' (coach is a kind of trainer).
's'+'low'+'coach'='SLOWCOACH'
(Other definitions for slowcoach that I've seen before include "Not a rapid operator" , "Laggard" , "Snail" , "trailer" , "plodder" .)