Ship in the past taking king aboard (4)
I believe the answer is:
argo
'ship' is the definition.
(ship in Greek myth)
'past taking king aboard' is the wordplay.
'past' becomes 'ago' (similar in meaning).
'taking king' becomes 'r' (abbreviation for rex, king in Latin. I am not sure about the 'taking' bit.).
'aboard' means one lot of letters goes inside another.
'ago' enclosing 'r' is 'ARGO'.
'in the' acts as a link.
(Other definitions for argo that I've seen before include "Old ship" , "Fabulous ship" , "Famous ship" , "Ship used by Jason in his search for the Golden Fleece" , "Stars once recognised in the south" .)