Picking on the large model, unknown one’s brought in (6)

'picking on the large' is the definition.
(discriminatory to larger people)

'model unknown one's brought in' is the wordplay.
'model' becomes 'sit' (e.g. for a painting).
'unknown' becomes 'z' (algebra).
'one's' becomes 'I's' (Roman numeral).
'brought in' means one lot of letters goes inside another.
'z'+'is'='zis'
'sit' placed around 'zis' is 'SIZIST'.