Small, almost entirely disgusting, bug (6)
I believe the answer is:
weevil
'bug' is the definition.
(weevil is a type of beetle)
'small almost entirely disgusting' is the wordplay.
'small' becomes 'wee' (Scottish term for small).
'almost entirely' means to remove the last letter.
'disgusting' becomes 'vile' (I've seen this in another clue).
'vile' with its last letter removed is 'vil'.
'wee'+'vil'='WEEVIL'
(Other definitions for weevil that I've seen before include "Small beetle, pest of crops" , "Small beetle that feeds on plants" , "Small beetle, crop pest" , "Type of beetle that feeds of stored grain etc" , "We live with small beetle that feeds on grain and tree-bark" .)