Only one tooth? A bit of a farce, simply useless! (4)
I believe the answer is:
fang
'only one tooth?' is the definition.
'fang' can be an answer for 'tooth?' (fang is a kind of tooth). I am not sure about the 'only one' bit.
'a bit of a farce simply useless' is the wordplay.
'a bit of' says to take the initial letters.
'simply useless' becomes 'ang' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The initial letter of 'farce' is 'f'.
'f'+'ang'='FANG'
Can you help me to learn more?
(Other definitions for fang that I've seen before include "Eg, wolf or snake's tooth" , "possible poisoner" , "Large pointed tooth" , "Venom-tooth" , "Snake tooth" .)