The former left for trade elsewhere (6)
I believe the answer is:
export
'trade elsewhere' is the definition.
'export' can be an answer for 'trade' (exporting is a kind of trading). I am not certain of the 'elsewhere' bit.
'the former left' is the wordplay.
'the former' becomes 'ex' (prefix meaning former or onetime).
'left' becomes 'port' (left side of a ship - opposite of starboard).
'ex'+'port'='EXPORT'
'for' is the link.
Can you help me to learn more?
(Other definitions for export that I've seen before include "Carry out of country" , "Send goods for sale abroad" , "Dispose of to another country" , "ship overseas" , "Sell goods abroad" .)