Centre, roughly around 100, is including Conservatives first, also the unconventional (10)

'the unconventional' is the definition.
(unconventional people)

'centre roughly around 100 is including conservatives first' is the wordplay.
'roughly' indicates an anagram.
'around' means one lot of letters goes inside another.
'c00' becomes 'C' (Roman numeral).
'including' means one lot of letters goes inside another.
'conservatives first' becomes 'c' (1st letter of 'conservatives').
'centre' anagrammed gives 'ecentr'.
'ecentr' going around 'c' is 'eccentr'.
'is' going around 'c' is 'ics'.
'eccentr'+'ics'='ECCENTRICS'

'also' acts as a link.