Little thing wriggling around in utero without name (8)
I believe the answer is:
neutrino
'little thing' is the definition.
(sub-atomic particle)
'wriggling around in utero without name' is the wordplay.
'wriggling around' indicates an anagram.
'without' means one lot of letters goes inside another ('without' can be similar in meaning to 'outside').
'name' becomes 'n' (common abbreviation e.g. on forms).
'in'+'utero'='inutero'
'inutero' is an anagram of 'neutrio'.
'neutrio' going around 'n' is 'NEUTRINO'.
(Other definitions for neutrino that I've seen before include "This barely affects anything" , "Subatomic particle, now said to travel faster than light" , "Uncharged elementary particle" , "Small piece of matter" , "Minute item lacking mass" .)