What sailor may follow follows sailor! (3,4)
I believe the answer is:
dog star
'what sailor may follow' is the definition.
(sailors might navigate using the Dog Star)
'follows sailor' is the wordplay.
'follows' becomes 'dogs' (dog can mean to follow).
'sailor' becomes 'tar' (slang term for a sailor).
'dogs'+'tar'='DOG-STAR'
(Other definitions for dog star that I've seen before include "Sirius may also be arts god" , "Part of 8 [CANIS MAJOR]" , "slightly flashy night-spot?" , "Sirius in the sky" , "clearly visible shiner" .)