Pest needs 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' indicates putting letters inside (some letters go outside others).
'pub' becomes 'ph' (abbreviation for Public House).
'aid' enclosing 'ph' is 'APHID'.
'needs' is the link.
(Other definitions for aphid that I've seen before include "Greenfly etc" , "Insect feeding on plant juices" , "insect pest" , "Horticultural pest" , "Garden bug" .)