Fairly short, though blooming early once! (5)
I believe the answer is:
rathe
'blooming early once' is the definition.
(archaic word meaning blooming early in the year)
'fairly short' is the wordplay.
'fairly' becomes 'rather' (I've seen this before).
'short' means to remove the last letter.
'rather' with its last letter removed is 'RATHE'.
'though' acts as a link.
(Other definitions for rathe that I've seen before include "prompt, in former times" , "Quick once" , "Blooming early" , "Eager" .)