Extremely low in tree-trunk, harmful beetle (4,6)
I believe the answer is:
boll weevil
'beetle' is the definition.
(boll weevil is a kind of beetle)
'extremely low in tree-trunk harmful' is the wordplay.
'extremely' suggests removing the centre.
'in' indicates putting letters inside.
'tree-trunk' becomes 'bole' (I've seen this before).
'harmful' becomes 'evil' ('evil' can be a synonym of 'harmful').
'low' with its middle taken out is 'lw'.
'lw' placed within 'bole' is 'bollwe'.
'bollwe'+'evil'='BOLL-WEEVIL'
(Another definition for boll weevil that I've seen is " Cotton pest".)