Young kangaroo who may now be holder of a bus pass (4,6)
I believe the answer is:
baby boomer
'who may now be holder of a bus pass' is the definition.
'young kangaroo' is the wordplay.
'young' becomes 'baby' (baby is a kind of young).
'kangaroo' becomes 'boomer' (I can't explain this - if you can you should believe this answer much more).
'baby'+'boomer'='BABY BOOMER'
(Other definitions for baby boomer that I've seen before include "Joey" , "Person born shortly after WWII" , "One of many born after WW2" , "born just after the war?" , "Someone born in the decade after WWII" .)