Worm, harmless coiling round, getting into knot (8)
I believe the answer is:
nematode
'worm' is the definition.
(nematode is a kind of worm)
'harmless coiling round getting into knot' is the wordplay.
'harmless' becomes 'tame'.
'coiling round' is a reversal indicator.
'getting into' is an insertion indicator.
'knot' becomes 'node' (both can mean a swollen lump).
'tame' reversed gives 'emat'.
'emat' placed inside 'node' is 'NEMATODE'.
(Other definitions for nematode that I've seen before include "Type of worm" , "sort of animal" , "Long thin worm such as threadworm" .)