Organ to cover an insect (6)
I believe the answer is:
earwig
'an insect' is the definition.
(I know that earwig is a type of insect)
'organ to cover' is the wordplay.
'organ' becomes 'ear' (the ear is an organ).
'to cover' becomes 'wig' (wig is a kind of cover).
'ear'+'wig'='EARWIG'
(Other definitions for earwig that I've seen before include "Insect with large rear pincers" , "Insect with antennae and pincers" , "creepy-crawly" , "Small insect with aural toupee?" , "Clawed insect" .)