Flower opening one spring evening, at first (3,4)
I believe the answer is:
tea rose
'flower' is the definition.
(I've seen this before)
'opening one spring evening at first' is the wordplay.
'opening' becomes 'tear' (tear is a kind of opening**).
'at first' indicates taking the first letters.
The first letters of 'one spring evening' is 'ose'.
'tear'+'ose'='TEA-ROSE'
(Other definitions for tea rose that I've seen before include "Eats roe (anag.)" , "Garden plant" , "Bloom" , "Flowering bush" , "Scented garden flower" .)