Shipping lanes are visibly affected, not initially heading westward (5)
I believe the answer is:
naval
'shipping' is the definition.
'lanes are visibly affected not initially heading westward' is the wordplay.
'initially' suggests taking the first letters.
'heading westward' is a reversal indicator.
The initial letters of 'lanes are visibly affected not' is 'lavan'.
'lavan' reversed gives 'NAVAL'.
(Other definitions for naval that I've seen before include "of fighting sailors" , "Of seaborne forces" , "Relating to warships" , "To do with the marine branch of an army" , "Relating to a nation's fleet" .)