Insect concealed under a plant's head (5)
I believe the answer is:
aphid
'insect' is the definition.
(aphid is a kind of insect)
'concealed under a plant's head' is the wordplay.
'concealed' becomes 'hid' ('hide' can be a synonym of 'conceal'**).
'under' is a charade indicator (letters next to each other).
'head' indicates taking the first letters.
The first letter of 'plant' is 'p'.
'a'+'p'='ap'
'hid' after 'ap' is 'APHID'.
(Other definitions for aphid that I've seen before include "plant pest" , "Fly" , "Sap-sucking bug" , "Horticultural pest" , "Small plant-sucking insect" .)