Port from which a UK sailor returns (4)
I believe the answer is:
baku
'port' is the definition.
(I've seen this before)
'a uk sailor returns' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'returns' is a reversal indicator.
'uk'+'ab'='ukab'
'ukab' reversed gives 'BAKU'.
'from which' acts as a link.
(Other definitions for baku that I've seen before include "Caspian Sea port" , "Capital of Azerbaijan on shore of Caspian Sea" , "Azerbaijan's capital" , "Capital city of Azerbaijan" .)