Plant - one in lab Leo rearranged (7)
I believe the answer is:
lobelia
'plant' is the definition.
(lobelia is a kind of plant)
'one in lab leo rearranged' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'rearranged' indicates anagramming the letters.
'lab'+'leo'='lableo'
'lableo' anagrammed gives 'lobela'.
'i' inserted into 'lobela' is 'LOBELIA'.
(Other definitions for lobelia that I've seen before include "Garden plant with five-lobed flowers" , "Tropical or subtropical bedding plant" , "Common garden plant; oil bale (anag.)" , "Lo, I bale out the flower" , "Bloomer" .)