Where there are grounds, they support the supporters (6)
I believe the answer is:
stands
'where there are grounds they' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'support the supporters' is the wordplay.
'support' becomes 'stand' (stand is a kind of support).
'the supporters' becomes 's' (this could be a standard abbreviation which I don't know about).
'stand'+'s'='STANDS'
Can you help me to learn more?
(Other definitions for stands that I've seen before include "Tiered seats in stadiums" , "Puts up with" , "Gets to one's feet" , "Stadium seats" , "doesn't change" .)