Some sap concealed a plant louse (5)
I believe the answer is:
aphid
'a plant louse' is the definition.
(aphid is a kind of plant louse)
'some sap concealed' is the wordplay.
'some sap' becomes 'ap' (I can't justify this - if you can you should give a lot more credence to this answer).
'concealed' becomes 'hid' ('hide' can be a synonym of 'conceal').
'ap'+'hid'='APHID'
Can you help me to learn more?
(Other definitions for aphid that I've seen before include "Greenfly etc" , "Sap-sucking bug" , "Fly" , "Small plant-sucking insect" , "Insect feeding on plant juices" .)