Talk fish up in miscellany (6)
I believe the answer is:
ragbag
'miscellany' is the definition.
(I know that ragbag is a type of miscellany)
'talk fish up' is the wordplay.
'talk' becomes 'gab' (I've seen this in another clue).
'fish' becomes 'gar' (gar is a kind of fish).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'gab'+'gar'='gabgar'
'gabgar' written backwards gives 'RAGBAG'.
'in' acts as a link.
(Other definitions for ragbag that I've seen before include "paperboy's satchel?" , "Hodge-podge" , "Motley collection" , "Motley assortment of things" , "Miscellany" .)