Prime minister's surreptitious bug (6)
I believe the answer is:
mayfly
'bug' is the definition.
(mayfly is a type of insect)
'prime minister's surreptitious' is the wordplay.
'prime minister' becomes 'may' (I've seen this in other clues).
'surreptitious' becomes 'fly' (fly can mean crafty or surreptitious).
'may'+'fly'='MAYFLY'
(Other definitions for mayfly that I've seen before include "Ephemeral insect" , "Short-lived spring insect" , "Short-lived insect with delicate wings" .)