What could attract a greater number to get back in ring (9)
I believe the answer is:
pheromone
'back in ring' is the definition.
The definition suggests a singular noun which matches the answer.
'what could attract a greater number' is the wordplay.
I cannot really see how this works, but
'a' could be 'one' and 'one' is found within the answer.
'number' could be 'm' (M is an example) and 'm' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
'to get' acts as a link.
Can you help me to learn more?
(Other definitions for pheromone that I've seen before include "What attracts" , "Chemical signal between organisms" , "Chemical attractor" , "Substance secreted that affects others of same species" , "Chemical affecting behaviour" .)