Figure in Tonga excited by backing of company (7)
I believe the answer is:
octagon
'figure' is the definition.
(type of geometrical figure)
'tonga excited by backing of company' is the wordplay.
'excited' indicates anagramming the letters.
'by' means one lot of letters go next to another.
'backing of' shows that the letters should be reversed in order.
'company' becomes 'co' (abbreviation).
'co' back-to-front is 'oc'.
'tonga' with letters rearranged gives 'tagon'.
'tagon' after 'oc' is 'OCTAGON'.
'in' is the link.
(Other definitions for octagon that I've seen before include "with eight angles" , "Geometric figure with eight sides" , "Eight-sided shape" , "Eight sided figure" , "Figure of many sides" .)