I am presuming your are referring to clause (vii) of Section 24 of the CGST Act, 2017 which reads as follows:
"persons who make taxable supply of goods or services or both on behalf of other taxable persons whether as an agent or otherwise;"
I am further presuming that your feel that as real estate broker is acting as "agent', he is liable for compulsory registration in above-said clause.
If my presumptions are correct, in my view, such real estate broker is NOT liable for compulsory registration till his aggregate turnover reaches Rs. 20 Lac (or Rs. 10 Lac, depending upon state / UT) in a year.
Even though they are acting as 'broker / agent', 'they are really not making taxable supply of goods or services or both on behalf of other taxable persons whether as an agent or otherwise' while performing your business activities (i.e. for facilitating agreement between landlord and tenants / for facilitating selling of properties between two parties, as real estate broker).
In other words, 'Real estate broker / agent' are not giving property on rent on some else's behalf or selling / buying property on some else's behalf.
In my view, these services are provided by 'Real estate broker / agent' on their own behalf and not on behalf of any other person. Hence, said Section 24 (vii) does not come into play.
I could not link definition given u/s 2 (15) with context of your query.
If I am mistaken in my presumptions or context of your doubts / query etc., request you to elaborate & clarify your query further.
All above are strictly personal views of mine and the same should not be construed as professional advice / suggestion.