Leading light left before British sailors (7)
I believe the answer is:
lantern
'leading light' is the definition.
(I've seen this before)
'left before british sailors' is the wordplay.
'left' becomes 'l' (common abbreviation).
'before' becomes 'ante' (prefix meaning 'before').
'british sailors' becomes 'RN' (Royal Navy).
'l'+'ante'+'rn'='LANTERN'
(Other definitions for lantern that I've seen before include "Light-admitting structure on roof" , "It illuminates" , "Enclosed lamp" , "Type of lamp" , "Transparent case for candle" .)