Scoffing fish sandwiches father served up (8)
I believe the answer is:
derisory
'scoffing' is the definition.
(I've seen this before)
'fish sandwiches father served up' is the wordplay.
'fish' becomes 'dory' (dory is a kind of fish).
'sandwiches' means one lot of letters goes inside another (inserted letters are sandwiched in).
'father' becomes 'sire' (synonyms).
'served up' shows that the letters should be reversed in order (in a down clue, letters go up).
'sire' back-to-front is 'eris'.
'dory' enclosing 'eris' is 'DERISORY'.
(Other definitions for derisory that I've seen before include "pathetically inadequate?" , "worthy of scorn?" , "Showing scorn or ridicule" , "ridiculously little" , "Ridiculously inadequate" .)