Ultimately useless dairy product -- that's a laugh! (6)
I believe the answer is:
scream
'that's a laugh' is the definition.
(scream is a kind of laugh)
'ultimately useless dairy product' is the wordplay.
'ultimately' suggests the final letters.
'dairy product' becomes 'cream' (cream is a kind of dairy product).
The last letter of 'useless' is 's'.
's'+'cream'='SCREAM'
(Other definitions for scream that I've seen before include "Cry of anguish" , "Assault victim's reaction, possibly" , "cry loudly" , "one causes great amusement" , "funny man" .)