Heavy weight old train firm transports (7)
I believe the answer is:
bouncer
'heavy' is the definition.
(both can mean a member of security staff)
'weight old train firm transports' is the wordplay.
I cannot really understand how this works, but
'weight' could be 'ounce' (ounce is a kind of weight) and 'ounce' is found in the answer.
'old train firm' could be 'br' (British Rail) and 'br' is present in the leftover letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for bouncer that I've seen before include "trampolinist?" , "Nightclub security doorman" , "tough doorman" , "A heavy - ball, say" , "Chucker-out for nightclub" .)