A cost involved over a black pepper sauce (7)
I believe the answer is:
tabasco
'pepper sauce' is the definition.
(I've seen this before)
'a cost involved over a black' is the wordplay.
'involved' indicates an anagram (involved can mean confusing or complex).
'over' means one lot of letters goes inside another.
'black' becomes 'b' (as in pencil classifications**).
'a'+'cost'='acost'
'acost' with letters rearranged gives 'tasco'.
'a'+'b'='ab'
'tasco' enclosing 'ab' is 'TABASCO'.
(Other definitions for tabasco that I've seen before include "Hot red sauce" , "Mexico state" , "Hot sauce made from capsicums" , "condiment?" , "ASBO Act (anag) - hot stuff!" .)