In the main, it’s a great leveller (10)
I believe the answer is:
stabiliser
'a great leveller' is the definition.
The definition suggests a singular noun which matches the answer.
'in the main it's' is the wordplay.
I cannot quite understand how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is present in the answer.
'it' could be 'sa' (SA can mean sex appeal, as in 'it') and 'sa' is present in the remaining letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for stabiliser that I've seen before include "Device reducing rolling of ship" , "Tailplane" .)