Sparkling harangue securing upturn in support (7)
I believe the answer is:
radiant
'sparkling' is the definition.
(I've seen this in another clue)
'harangue securing upturn in support' is the wordplay.
'harangue' becomes 'rant' (synonyms).
'securing' means one lot of letters goes inside another.
'upturn in' shows that the letters should be reversed in order.
'support' becomes 'aid' (to aid is to help or support).
'aid' in reverse letter order is 'dia'.
'rant' enclosing 'dia' is 'RADIANT'.
(Other definitions for radiant that I've seen before include "Very rich (7.2.2)" , "Shining brightly, being at nadir" , "Effulgent" , "Incandescent" , "Glowing with joy" .)