Inventor a large group backed (5)
I believe the answer is:
tesla
'inventor' is the definition.
(I've seen this before)
'a large group backed' is the wordplay.
'a large' becomes 'al' (I've seen this in other clues).
'group' becomes 'set'.
'backed' shows that the letters should be reversed in order.
'al'+'set'='alset'
'alset' in reverse letter order is 'TESLA'.
(Other definitions for tesla that I've seen before include "electrical unit" , "Physicist" , "Nikola ---, electricity pioneer" , "Unit of magnetic flux density" , "Inventor" .)