It provides no latitude for the navigator (7)
I believe the answer is:
equator
'it provides no latitude' is the definition.
The definition suggests a singular noun which matches the answer.
'for the navigator' is the wordplay.
I cannot quite understand how this works, but
'for' could be 'to' (eg both can mean 'in the opinion of') and 'to' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for equator that I've seen before include "It's at zero degrees latitude" , "Circle round the globe" , "Hemisphere divider" , "Imaginary circle round the earth" , "A circle around earth's centre" .)