Subject to snap (5)
I believe the answer is:
topic
'subject' is the definition.
('topic' can be a synonym of 'subject')
'to snap' is the wordplay.
'snap' becomes 'pic'.
'to'+'pic'='TOPIC'
(Other definitions for topic that I've seen before include "Subject matter, theme" , "might this be hot?" , "A subject for conversation or debate" , "Something to discuss" , "Question" .)