Perhaps shorter trains derail, losing their rear halves (6)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

TRA [ins] DER [ail] with the rear half of each word dropped. Trader = someone who buys and sells on the stock exchange; short, as a verb = to sell stocks that are expected to fall in value, with the intention of making a profit by buying them back later at a lower price.