Transport king to sing outside station? (4)
I believe the answer is:
busk
'sing outside station?' is the definition.
(to busk is to perform for money in a public place)
'transport king' is the wordplay.
'transport' becomes 'bus' (bus is a kind of transport).
'king' becomes 'k' (abbreviation for king in chess).
'bus'+'k'='BUSK'
'to' is the link.
(Other definitions for busk that I've seen before include "Play an instrument on the street for money" , "Perform music in the street for money" , "Play street music" , "Prepare" , "Perform, seeking coins" .)