It beat time, a guide when one's at sea (7)
I believe the answer is:
sextant
'it beat time a guide when one's at sea' is the definition.
The definition suggests a singular noun which matches the answer.
'it beat time' is the wordplay.
'it' becomes 'sex' (I can't explain this - if you can you should believe this answer much more).
'beat' becomes 'tan' (I've seen this before).
'time' becomes 't'.
'sex'+'tan'+'t'='SEXTANT'
Can you help me to learn more?
(Other definitions for sextant that I've seen before include "Navigator's instrument" , "Mariner's instrument" , "Measuring instrument for navigation" , "Navigational aid" , "Navigational instrument for finding position" .)