Supposition destroyed her toy (6)
I believe the answer is:
theory
'supposition' is the definition.
(I've seen this before)
'destroyed her toy' is the wordplay.
'destroyed' is an anagram indicator.
'her'+'toy'='hertoy'
'hertoy' with letters rearranged gives 'THEORY'.
(Other definitions for theory that I've seen before include "Proposition, thesis" , "Principle on which practice is based" , "Hypothesis or concept" , "Supposition or idea" , "Unproven explanation" .)