Worst all over — upper-class girl kidnapped by parliamentarian (8)

'worst' is the definition.
('pessimum' can be similar in meaning to 'worst')

'all over upper-class girl kidnapped by parliamentarian' is the wordplay.
'all over' is a reversal indicator.
'upper-class' becomes 'u' ('u' can be similar in meaning to 'upper-class').
'girl' becomes 'miss' (I've seen this before**).
'kidnapped by' means one lot of letters goes inside another.
'parliamentarian' becomes 'mep' (member of the european parliament).
'u'+'miss'='umiss'
'umiss' placed within 'mep' is 'mumissep'.
'mumissep' back-to-front is 'PESSIMUM'.