Source of terrible cheek about high-class plant (5)
I believe the answer is:
tulip
'plant' is the definition.
The answer is an example.
'source of terrible cheek about high-class' is the wordplay.
'source of' suggests taking the first letters (the start - or source - of the word).
'cheek' becomes 'lip' (both can mean insolence or rudeness).
'about' means one lot of letters go next to another.
'high class' becomes 'U' (as in U and non-U speech).
The first letter of 'terrible' is 't'.
'lip' put after 'u' is 'ulip'.
't'+'ulip'='TULIP'
(Other definitions for tulip that I've seen before include "Liliaceous bulb" , "Plant at the centre of the Dutch financial bubble of 1637" , "Bulb producing a single showy flower" , "Spring bulb" , "It blooms in 11 down" .)