Maps to aid navigation (6)
I believe the answer is:
charts
'maps to aid navigation' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for charts that I've seen before include "Maps to aid navigation" , "Maps to assist navigation by air or sea" , "Maps; plans" , "Sheets of tables, graphs or diagrams" , "Maps for captains" .)