Flower one’s seen in the light (5)
I believe the answer is:
daisy
'flower' is the definition.
(daisy is a kind of flower)
'one's seen in the light' is the wordplay.
'one's' becomes 'is' (roman numeral).
'seen in' means one lot of letters goes inside another.
'the light' becomes 'day' (I've seen this in other clues).
'is' inserted within 'day' is 'DAISY'.
(Other definitions for daisy that I've seen before include "chain, just the smallest bit?" , "Common plant" , "Plant; girl's name" , "Common wildflower" , "Grassland flower" .)