Again, transport can be a problem (5)
I believe the answer is:
rebus
'a problem' is the definition.
(type of puzzle or problem)
'again transport' is the wordplay.
'again' becomes 're'.
'transport' becomes 'bus' (bus is a kind of transport).
're'+'bus'='REBUS'
'can be' acts as a link.
(Other definitions for rebus that I've seen before include "Visual wordplay - Rankin's inspector" , "Pictorial teaser" , "In Edinburgh, busy" , "that's puzzling" , "Puzzling symbols, etc" .)