Pontificate about source of rapid growth (6)
I believe the answer is:
sprout
'growth' is the definition.
(I've seen this before)
'pontificate about source of rapid' is the wordplay.
'pontificate' becomes 'spout' (I've seen this in another clue).
'about' means one lot of letters goes inside another.
'source of rapid' becomes 'r' (I can't explain this - if you can you should believe this answer much more).
'spout' going around 'r' is 'SPROUT'.
Can you help me to learn more?
(Other definitions for sprout that I've seen before include "Grow; vegetable" , "Grow like Proust" , "Small cabbage from Brussels?" , "Produce buds" , "Burgeon" .)