Pest wants assistance outside pub (5)
I believe the answer is:
aphid
'pest' is the definition.
(aphid is a type of pest)
'assistance outside pub' is the wordplay.
'assistance' becomes 'aid' (synonyms).
'outside' means one lot of letters goes inside another.
'pub' becomes 'ph' (public house).
'aid' going around 'ph' is 'APHID'.
'wants' acts as a link.
(Other definitions for aphid that I've seen before include "Sap-sucking bug" , "Garden pest" , "Insect feeding on plant juices" , "Plant louse" , "one living on juices" .)