More crowded transport that is run (6)
I believe the answer is:
busier
'more crowded' is the definition.
(busy can mean crowded)
'transport that is run' is the wordplay.
'transport' becomes 'bus' (bus is a kind of transport).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'run' becomes 'r' (cricket abbreviation).
'bus'+'ie'+'r'='BUSIER'
(Other definitions for busier that I've seen before include "More fully engaged or occupied" , "More full of activity" , "Having a great deal more to do" , "More active or engaged in work" , "More fully engaged in work" .)