In one drop, rain oddly glittering (10)
I believe the answer is:
iridescent
'glittering' is the definition.
(I know this)
'in one drop rain oddly' is the wordplay.
'in' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'drop' becomes 'descent' (I've seen this before).
'oddly' indicates alternate letters (letters in the odd-numbered positions are used).
The alternate letters of 'rain' are 'ri'.
'i'+'descent'='idescent'
'idescent' placed around 'ri' is 'IRIDESCENT'.
(Other definitions for iridescent that I've seen before include "Sparkling" , "Eye-catching" , "Like a rainbow" , "changing colour" , "with a glittering display" .)