Main bridge? (4)
I believe the answer is:
arch
I believe this is a double definition.
'main' is the first definition.
(I've seen this before)
'bridge?' is the second definition.
(thesaurus)
(Other definitions for arch that I've seen before include "Cunning; bridge" , "Wellington's, perhaps" , "Construction with a keystone" , "Knowing, sly" , "Consciously or affectedly playful - under a bridge?" .)