Collection of planets: Mars and — yes — lots spinning (5,6)
I believe the answer is:
solar system
'collection of planets' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'mars and yes lots spinning' is the wordplay.
'and' means one lot of letters go next to another.
'spinning' is an anagram indicator.
'yes'+'lots'='yeslots'
'mars' put after 'yeslots' is 'yeslotsmars'.
'yeslotsmars' with letters rearranged gives 'SOLAR SYSTEM'.
Can you help me to learn more?
(Other definitions for solar system that I've seen before include "Bodies around heater" , "What's held together by the Sun" , "Sun, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto" , "Sun and the planets collectively" , "Arrangement of planets around a sun" .)