Mainly according to the leader? (7)
I believe the answer is:
chiefly
'mainly' is the definition.
(I know that mainly can be written as chiefly)
'according to the leader?' is the wordplay.
'according' becomes 'ly' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'to' means one lot of letters go next to another (I've seen this in other clues).
'the leader?' becomes 'chief' (chief is a kind of leader).
'ly' after 'chief' is 'CHIEFLY'.
Can you help me to learn more?
(Other definitions for chiefly that I've seen before include "Mainly, principally" , "In the main" , "For the most part" .)