My boss, being sat on by geezer, is shocked! (5)

'shocked' is the definition.
Although both the answer and definition are past participle verbs, I cannot see how they can define each other.

'my boss being sat on by geezer' is the wordplay.
'my boss' becomes 'ed' (abbreviation for editor, the setter's boss).
'being sat on by' means one lot of letters go next to another.
'geezer' becomes 'man' (I've seen this before).
'ed' put after 'man' is 'MANED'.

'is' acts as a link.

Can you help me to learn more?