Heartless snake concealed bug (5)
I believe the answer is:
aphid
'bug' is the definition.
(I've seen this before)
'heartless snake concealed' is the wordplay.
'heartless' means to remove the middle letters.
'snake' becomes 'asp' (asp is a kind of snake).
'concealed' becomes 'hid' ('hide' can be a synonym of 'conceal').
'asp' with its middle removed is 'ap'.
'ap'+'hid'='APHID'
(Other definitions for aphid that I've seen before include "Horticultural pest" , "insect pest" , "Blackfly or greenfly" , "Sap-sucking bug" , "plant pest" .)