Subject of toy piece oddly missing (5)
I believe the answer is:
topic
'subject' is the definition.
(as in a topic for discussion)
'toy piece oddly missing' is the wordplay.
'toy' becomes 'top' (top is a kind of toy).
'oddly missing' indicates alternate letters (letters in odd-numbered positions removed).
The alternate letters of 'piece' are 'ic'.
'top'+'ic'='TOPIC'
'of' acts as a link.
(Other definitions for topic that I've seen before include "Subject matter, theme" , "Content of argument" , "Subject matter of a conversation or discussion" , "Issue (for discussion)" , "A subject for conversation or debate" .)