They instigate every other merger in this country (10)
I believe the answer is:
ringleader
'they instigate every other merger in this country' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'in this country' is the wordplay.
I cannot really understand how this works, but
'in' is within the answer.
'country' could be 'lea' (I've seen this before) and 'lea' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for ringleader that I've seen before include "Chief mischief maker" , "One heading up illicit activity" , "brains" , "Chief mischief-maker" , "Main villain" .)