Company register on a part of plant (7)
I believe the answer is:
corolla
'part of plant' is the definition.
(I've seen this before)
'company register on a' is the wordplay.
'company' becomes 'co' (abbreviation).
'register' becomes 'roll' (list of names).
'on' means one lot of letters go next to another.
'co'+'roll'+'a'='COROLLA'
(Other definitions for corolla that I've seen before include "part of plant" , "Internal whorl of a flower" , "Flower's petals collectively" , "bit of a bloomer" , "Whorl of flower petals" .)