Insect resorted to this mostly, swallowing bug (5,4)
I believe the answer is:
tiger moth
'insect' is the definition.
(tiger moth is a kind of insect)
'resorted to this mostly swallowing bug' is the wordplay.
'resorted' is an anagram indicator.
'mostly' means to remove the last letter.
'swallowing' indicates putting letters inside.
'bug' becomes 'germ' ('germ' can be a synonym of 'bug'**).
'to'+'this'='tothis'
'tothis' with its last letter taken away is 'tothi'.
'tothi' is an anagram of 'tioth'.
'tioth' enclosing 'germ' is 'TIGER-MOTH'.
(Other definitions for tiger moth that I've seen before include "this one has brightly-coloured wings" , "Woolly bear" , "Spotted flyer" , "Insect - aeroplane" , "producer of woolly bears" .)