Spoon out extreme bits found in nasal cavity (4)
I believe the answer is:
snot
'found in nasal cavity' is the definition.
(snot is found in the nose)
'spoon out extreme bits' is the wordplay.
'spoon' becomes 'sn' (I can't explain this - if you can you should give a lot more credence to this answer).
'extreme bits' means to remove the middle letters (only the letters on the extremes are used).
'out' with its middle removed is 'ot'.
'sn'+'ot'='SNOT'
(Other definitions for snot that I've seen before include "Dog" , "Mucus" , "Contemptible type" .)