Deep part of ocean or river surrounded by fish (6)
I believe the answer is:
trench
'deep' is the definition.
(I know that deep can be written as trench)
'ocean or river surrounded by fish' is the wordplay.
I cannot quite see how this works, but
'river' could be 'r' (abbreviation for river) and 'r' is found within the answer.
'fish' could be 'tench' (tench is a kind of fish) and 'tench' is found in the remaining letters.
No letters remain.
This explanation may well be incorrect...
'part of' is the link.
(Other definitions for trench that I've seen before include "at the front" , "Military cut in the earth" , "A deep ditch, as dug for troops" , "Long narrow ditch" , "It may be slit" .)