Left out, in flower and growing rapidly (7)
I believe the answer is:
booming
'growing rapidly' is the definition.
'booming' can be an answer for 'growing' (booming is a kind of growing). I am unsure of the 'rapidly' bit.
'left out in flower' is the wordplay.
'left' becomes 'l' (common abbreviation).
'out' is a deletion indicator.
'in flower' becomes 'blooming' (I've seen this before).
'blooming' with 'l' taken out is 'BOOMING'.
'and' is the link.
Can you help me to learn more?
(Other definitions for booming that I've seen before include "as 26 [BITTERN] may be" , "Doing well financially" .)