Pest turned up every year then disappeared (5)
I believe the answer is:
aphid
'pest' is the definition.
(aphid is a type of pest)
'turned up every year then disappeared' is the wordplay.
'turned up' says the letters should be written backwards.
'every year' becomes 'pa' (per annum).
'then' says to put letters next to each other.
'disappeared' becomes 'hid' (I can't explain this - if you can you should believe this answer much more).
'pa' back-to-front is 'ap'.
'ap'+'hid'='APHID'
Can you help me to learn more?
(Other definitions for aphid that I've seen before include "Insect feeding on plant juices" , "Greenfly etc" , "Plant louse" , "Sap-sucking bug" , "Garden pest" .)