Aphid firstly, then bee, appear in flower (6)
I believe the answer is:
abloom
'in flower' is the definition.
(I've seen this before)
'aphid firstly then bee appear' is the wordplay.
'firstly' suggests taking the first letters.
'then' says to put letters next to each other.
'bee' becomes 'b'.
'appear' becomes 'loom' (looming is a kind of appearing**).
The initial letter of 'aphid' is 'a'.
'a'+'b'+'loom'='ABLOOM'
(Other definitions for abloom that I've seen before include "Flourishing" , "In flower" , "no longer buddy of Rose?" , "covered in flowers?" , "Flowering" .)