According to Spooner, delphinium is little gem (8)

I believe the answer is:

sparkler

I'm a little stuck... Click here to teach me more about this clue!

'gem' is the definition.
(I've seen this before)

I don't understand how the remainder of the clue works.

(Other definitions for sparkler that I've seen before include "Hand-held firework" , "Diamond (colloq.)" , "Eg, a diamond" , "Jewel" , "Slow-burning firework and a diamond" .)

I've seen this clue in The Independent.

Want a hint initially instead of a full solution? Install my app