River ultimately where small fish live (6)
I believe the answer is:
reside
'live' is the definition.
(residing is a kind of living)
'river ultimately where small fish' is the wordplay.
'river' becomes 'r'.
'ultimately' suggests the final letters.
'small' becomes 's' (abbreviation - e.g. clothes size).
'fish' becomes 'ide' (I've seen this before).
The last letter of 'where' is 'e'.
'r'+'e'+'s'+'ide'='RESIDE'
(Other definitions for reside that I've seen before include "Dwell in desire" , "Stay - desire (anag)" , "Dwell - settle" , "'Live, dwell (6)'" , "Live (in a place)" .)