Pest wants help outside pub (5)
I believe the answer is:
aphid
'pest' is the definition.
(aphid is a type of pest)
'help outside pub' is the wordplay.
'help' becomes 'aid' (synonyms).
'outside' means one lot of letters goes inside another.
'pub' becomes 'ph' (abbreviation for Public House).
'aid' going around 'ph' is 'APHID'.
'wants' is the link.
(Other definitions for aphid that I've seen before include "Plant louse" , "little sucker" , "Small insect like greenfly" , "one living on juices" , "Plant-louse" .)