Little ones may have no rims (6)
I believe the answer is:
minors
'rims' is the definition.
Although both the answer and definition are plural nouns, I don't see how one could define the other.
'little ones may have no' is the wordplay.
'little ones may' becomes 'mirs' (I can't justify this - if you can you should give a lot more credence to this answer).
'have' means one lot of letters goes inside another.
'mirs' enclosing 'no' is 'MINORS'.
Can you help me to learn more?
(Other definitions for minors that I've seen before include "Young people" , "They are legally under-age" , "They're too young to vote" , "could be little ones" , "They're under-age" .)