Formerly it was the way to go (4)
I believe the answer is:
exit
'the way to go' is the definition.
(I've seen this in another clue)
'formerly it' is the wordplay.
'formerly' becomes 'ex' (prefix meaning former or onetime).
'ex'+'it'='EXIT'
'was' acts as a link.
(Other definitions for exit that I've seen before include "Act of going out" , "Outlet" , "Player's departure" , "walk away" , "Way to leave" .)