Tranquil, in the main? (7)
I believe the answer is:
pacific
This clue is a double definition.
'tranquil' is the first definition.
(I've seen this in another clue)
'the main?' is the second definition.
(Pacific is an example)
'in' acts as a link.
(Other definitions for pacific that I've seen before include "Peaceful in character" , "Main" , "Peaceable" , "steam locomotive?" , "Largest ocean" .)