Toy in pile at worst malfunctioning (5,6)
I believe the answer is:
water pistol
'toy' is the definition.
(I know that water pistol is a type of toy)
'pile at worst malfunctioning' is the wordplay.
'malfunctioning' indicates anagramming the letters.
'pile'+'at'+'worst'='pileatworst'
'pileatworst' anagrammed gives 'WATER-PISTOL'.
'in' acts as a link.
(Other definitions for water pistol that I've seen before include "Toy weapon" , "Source of aggressive jet" , "Toy firearm" , "Plaything" , "Small bomb" .)