Crowded onto the bus, yapping (4)
I believe the answer is:
busy
'crowded' is the definition.
(I've seen this before)
'onto the bus yapping' is the wordplay.
'onto' is a charade indicator (letters next to each other).
'yapping' becomes 'y' (this could be a standard abbreviation of which I'm unaware).
'bus' next to 'y' is 'BUSY'.
Can you help me to learn more?
(Other definitions for busy that I've seen before include "Pot plant" , "Active (as a bee?)" , "Full of activity" , "Without leisure" , "With much to do" .)