Excessively complex set containing one digit (7)
I believe the answer is:
tootsie
'digit' is the definition.
(I've seen this before)
'excessively complex set containing one' is the wordplay.
'excessively' becomes 'too' (to too great a degree).
'complex' indicates anagramming the letters.
'containing' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'set' with letters rearranged gives 'tse'.
'tse' enclosing 'i' is 'tsie'.
'too'+'tsie'='TOOTSIE'
(Other definitions for tootsie that I've seen before include "Film with Dustin Hoffman" , "Foot or toe - I set, too (anag)" , "A child's foot?" , "pretty girl" , "Funnily enough, foot" .)