D'Artagnan finally brought up "All for one, and one..." — a bloomer (7)
I believe the answer is:
nigella
'a bloomer' is the definition.
(nigella is a kind of flowering plant)
'd'artagnan finally brought up all for one and one' is the wordplay.
'finally' suggests the final letters.
'brought up' says the letters should be written in reverse (in a down clue, letters go up).
'for one' becomes 'eg' (both can mean 'for example').
'and' is a charade indicator (letters next to each other).
'one' becomes 'i' (Roman numeral).
The final letter of 'dartagnan' is 'n'.
'all'+'eg'='alleg'
'alleg' backwards is 'gella'.
'gella' after 'i' is 'igella'.
'n'+'igella'='NIGELLA'
(Other definitions for nigella that I've seen before include "Flower and TV chef" , "plant" , "Flower and tv cook" , "Woman" , "girl" .)