Pest a party leader concealed (5)
I believe the answer is:
aphid
'pest' is the definition.
(aphid is a type of pest)
'a party leader concealed' is the wordplay.
'party leader' becomes 'p' (leader of the word 'party').
'concealed' becomes 'hid' ('hide' can be a synonym of 'conceal').
'a'+'p'+'hid'='APHID'
(Other definitions for aphid that I've seen before include "Plant-louse" , "Garden bug" , "Blackfly or greenfly" , "one living on juices" , "Horticultural pest" .)