Some lunatic zealous order initially in charge of an era (8)

'an era' is the definition.
(I know that Mesozoic is a era)

'some lunatic zealous order initially in charge' is the wordplay.
'lunatic' is an anagram indicator.
'initially' says to take the initial letters.
'in charge' becomes 'ic' (abbreviation for 'in charge').
The initial letters of 'zealous order' is 'zo'.
'some' is an anagram of 'meso'.
'meso'+'zo'+'ic'='MESOZOIC'

'of' is the link.