Application shown by a police constable in tin hat (8)

'application shown' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.

'a police constable in tin hat' is the wordplay.
'police constable' becomes 'pc'.
'in' means one lot of letters goes inside another.
'tin' becomes 'Sn' (Sn is the chemical symbol for tin).
'a'+'pc'='apc'
'sn'+'hat'='snhat'
'apc' put within 'snhat' is 'SNAPCHAT'.

'by' acts as a link.

Can you help me to learn more?