A mocking 'Fish, sire, swim in it' (8)
I believe the answer is:
derisory
'a mocking' is the definition.
(thesaurus)
'fish sire swim in it' is the wordplay.
'fish' becomes 'dory' (dory is a kind of fish).
'swim' is an anagram indicator.
'in it' means one lot of letters goes inside another.
'sire' with letters rearranged gives 'eris'.
'dory' going around 'eris' is 'DERISORY'.
(Other definitions for derisory that I've seen before include "Deserving of ridicule, laughable" , "Showing scorn or ridicule" , "Ludicrous" , "Contemptible" , "Pathetic" .)