Pest given assistance outside public house (5)
I believe the answer is:
aphid
'pest' is the definition.
(aphid is a type of pest)
'assistance outside public house' is the wordplay.
'assistance' becomes 'aid' (synonyms).
'outside' is an insertion indicator (some letters go outside others).
'public house' becomes 'ph' (abbreviation).
'aid' enclosing 'ph' is 'APHID'.
'given' acts as a link.
(Other definitions for aphid that I've seen before include "Insect feeding on plant juices" , "Small insect like greenfly" , "Plant-louse" , "Garden bug" , "little sucker" .)