A plant initially concealed a plant pest (5)
I believe the answer is:
aphid
'a plant pest' is the definition.
(I've seen this before)
'a plant initially concealed' is the wordplay.
'initially' says to take the initial letters.
'concealed' becomes 'hid' ('hide' can be a synonym of 'conceal'**).
The initial letter of 'plant' is 'p'.
'a'+'p'+'hid'='APHID'
(Other definitions for aphid that I've seen before include "Garden pest" , "Insect feeding on plant juices" , "Horticultural pest" , "Sap-sucking bug" , "Garden bug" .)