Small girl getting on for some fun on wheels (13)
I believe the answer is:
skateboarding
'some fun on wheels' is the definition.
(skateboards have wheels)
'small girl getting on for' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'girl' becomes 'Kate' (girl's name).
'getting on for' becomes 'boarding' (I can't explain this - if you can you should believe this answer much more).
's'+'kate'+'boarding'='SKATEBOARDING'
Can you help me to learn more?
(Other definitions for skateboarding that I've seen before include "Moving using strip of wood on wheels" , "Teenagers' pastime" , "Wheeled recreation" , "sport" , "recreational activity" .)