Unit is able to lead off (7)
I believe the answer is:
candela
'unit' is the definition.
(candela is a kind of unit)
'able to lead off' is the wordplay.
'able' becomes 'can'.
'to' is a charade indicator (letters next to each other).
'off' indicates anagramming the letters.
'lead' is an anagram of 'dela'.
'can'+'dela'='CANDELA'
'is' acts as a link.
(Other definitions for candela that I've seen before include "Measure of brilliance" , "Unit of luminous intensity" , "measure of light" .)