Toy gun found on top of this (6)
I believe the answer is:
trifle
'toy' is the definition.
(thesaurus)
'gun found on top of this' is the wordplay.
'gun' becomes 'rifle' (rifle is a kind of gun).
'found on' is a charade indicator (letters next to each other).
'top of' indicates taking the first letters.
The first letter of 'this' is 't'.
'rifle' put after 't' is 'TRIFLE'.
(Other definitions for trifle that I've seen before include "Filter (anag.)" , "Pudding - trinket" , "Mess" , "Insignificant matter" , "Touch" .)