In the morning, tiny bit of amanatsu zest is fantastic (7)
I believe the answer is:
amazing
'fantastic' is the definition.
(I've seen this before)
'in the morning tiny bit of amanatsu zest' is the wordplay.
'in the morning' becomes 'am'.
'tiny bit of' suggests taking the first letters.
'zest' becomes 'zing' (I've seen this before).
The first letter of 'amanatsu' is 'a'.
'am'+'a'+'zing'='AMAZING'
'is' acts as a link.
(Other definitions for amazing that I've seen before include "Incredible" , "Sensational" , "Astonishing, awesome" , "Filling with wonder, a huge surprise" , "Very impressive" .)