Dark web ultimately involved in transaction (5)
I believe the answer is:
sable
'dark' is the definition.
(I've seen this before)
'web ultimately involved in transaction' is the wordplay.
'ultimately' suggests the final letters.
'involved in' indicates putting letters inside.
'transaction' becomes 'sale' (sales are a type of transaction).
The final letter of 'web' is 'b'.
'b' going inside 'sale' is 'SABLE'.
(Other definitions for sable that I've seen before include "fur coat" , "Fur from member of weasel family" , "Dark brown fur of the marten" , "Black (heraldry); fur" , "Literary black" .)