Mounting a stage that is wide, making a bloomer (7)
I believe the answer is:
weigela
'a bloomer' is the definition.
(type of blooming plant)
'mounting a stage that is wide' is the wordplay.
'mounting' shows that the letters should be reversed in order (going up in a down clue).
'stage' becomes 'leg' (a leg is a stage of a competition).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'wide' becomes 'w' (abbreviation e.g. in cricket).
'a'+'leg'+'ie'+'w'='alegiew'
'alegiew' reversed gives 'WEIGELA'.
'making' acts as a link.
(Another definition for weigela that I've seen is " Ornamental shrub named after German botanist".)