Keen to play with toys — everything depends on it (8)
I believe the answer is:
keystone
'everything depends on it' is the definition.
I can't tell whether this defines the answer.
'keen to play with toys' is the wordplay.
'to play with' indicates an anagram.
'keen' mixed up with 'toys' gives 'KEYSTONE'.
Can you help me to learn more?
(Other definitions for keystone that I've seen before include "Chief brick in an arch" , "Central part of arch" , "Central policy; arch top" , "important element" , "Central principle of a policy" .)