Briefly spot partner making bit of a bloomer (5)
I believe the answer is:
sepal
'bit of a bloomer' is the definition.
(part of a flower)
'briefly spot partner' is the wordplay.
'briefly' means to remove the last letter.
'spot' becomes 'see' (both can mean to notice or observe).
'partner' becomes 'pal' (pal can mean a friend).
'see' with its last letter taken away is 'se'.
'se'+'pal'='SEPAL'
'making' acts as a link.
(Other definitions for sepal that I've seen before include "Lapse about part of a flower" , "Part of a flower - peals (anag)" , "Part of a flowering plant" , "Calyx segment" , "Part of a flower's outer envelope" .)