Worm boring around inside swelling (8)
I believe the answer is:
nematode
'worm' is the definition.
(I know that nematode is a type of worm)
'boring around inside swelling' is the wordplay.
'boring' becomes 'tame' (tame can mean dull or boring).
'around' says the letters should be written in reverse.
'inside' is an insertion indicator.
'swelling' becomes 'node' (swelling on the body).
'tame' reversed gives 'emat'.
'emat' put into 'node' is 'NEMATODE'.
(Other definitions for nematode that I've seen before include "Long thin worm such as threadworm" , "Type of worm" , "sort of animal" .)