Small mostly disgusting beetle (6)
I believe the answer is:
weevil
'beetle' is the definition.
(weevil is a kind of beetle)
'small mostly disgusting' is the wordplay.
'small' becomes 'wee' ('wee' can be similar in meaning to 'small').
'mostly' means to remove the last letter.
'disgusting' becomes 'vile' (I've seen this before).
'vile' with its final letter taken away is 'vil'.
'wee'+'vil'='WEEVIL'
(Other definitions for weevil that I've seen before include "Small beetle that feeds on plants" , "Evil we find strangely in a beetle" , "Small beetle with a long snout" , "Kind of beetle that damages crops" , "Insect that feeds on plant products" .)