Having come out, feel ultimately enthralled by a period of growth (6)
I believe the answer is:
abloom
'having come' is the definition.
I can't judge whether this definition defines the answer.
'feel ultimately enthralled by a period of growth' is the wordplay.
'ultimately' suggests the final letters.
'enthralled by' indicates putting letters inside ('enthral' can mean 'capture').
'period of growth' becomes 'boom' (I am not sure about this - if you are sure you should believe this answer much more).
The final letter of 'feel' is 'l'.
'a'+'boom'='aboom'
'l' going into 'aboom' is 'ABLOOM'.
'out' acts as a link.
Can you help me to learn more?
(Other definitions for abloom that I've seen before include "covered in flowers?" , "Flowering" , "In flower" , "Healthy" , "no longer buddy of Rose?" .)