Yellow loser (5)
I believe the answer is:
lemon
I believe this clue is a double definition.
'yellow' is the first definition.
(I know that lemon is a type of yellow)
'loser' is the second definition.
(thesaurus)
(Other definitions for lemon that I've seen before include "Defective object" , "Washout" , "Small evergreen fruit tree" , "Useless object" , "something defective" .)