Talk bringing decrease in fish up (6)
I believe the answer is:
yabber
'talk' is the definition.
'decrease in fish up' is the wordplay.
'decrease' becomes 'ebb' (ebb is a kind of decrease).
'in' means one lot of letters goes inside another.
'fish' becomes 'ray' (ray is a kind of fish).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'ebb' put into 'ray' is 'rebbay'.
'rebbay' in reverse letter order is 'YABBER'.
'bringing' is the link.
(Other definitions for yabber that I've seen before include "rabbit on" , "Conversation in Sydney" , "Chatter" .)