Follow street Arab, Ancient Mariner's guide? (3,4)
I believe the answer is:
dog star
'ancient mariner's guide?' is the definition.
(navigational aid for sailors)
'follow street arab' is the wordplay.
'follow' becomes 'dog' (I've seen this before).
'street' becomes 'st' (abbreviation).
'arab' becomes 'Ar.' (abbreviation in Chambers).
'dog'+'st'+'ar'='DOG-STAR'
(Other definitions for dog star that I've seen before include "Part of 8 [CANIS MAJOR]" , "'Another name for the Sirius constellation (3,4)'" , "heavenly body?" , "Sirius may also be arts god" , "clearly visible shiner" .)