Biting insect at the bottom of deep hole (9)
I believe the answer is:
trenchant
'biting' is the definition.
(I've seen this before)
'insect at the bottom of deep hole' is the wordplay.
'insect' becomes 'ant' (ant is a kind of insect).
'at the bottom of' means one lot of letters go next to another (in a down clue, some letters go below others).
'deep hole' becomes 'trench' (I am not sure about this - if you are sure you should believe this answer much more).
'ant' put after 'trench' is 'TRENCHANT'.
Can you help me to learn more?
(Other definitions for trenchant that I've seen before include "Sharp" , "Distinct" , "Clear" , "Fierce" , "Incisive in expression" .)