Fortify, across the pond, a private thoroughfare (4)
I believe the answer is:
gird
'fortify' is the definition.
('gird' can be a synonym of 'fortify')
'across the pond a private thoroughfare' is the wordplay.
'across the pond a private' becomes 'gi' (term for US soldier).
'thoroughfare' becomes 'rd' (abbreviation for road).
'gi'+'rd'='GIRD'
(Other definitions for gird that I've seen before include "Surround" , "Steel" , "Encircle - fasten with belt" , "Put a belt on" , "ring" .)