Location in which fish may be heard (5)
I believe the answer is:
place
'location' is the definition.
(a location is a place)
'fish may be heard' is the wordplay.
'fish' becomes 'plaice' (plaice is a kind of fish).
'may be heard' shows a homophone (sound like).
'plaice' sounds like 'PLACE'.
'in which' is the link.
(Other definitions for place that I've seen before include "vacancy" , "Venue" , "Locus" , "Identify -- station" , "Situation" .)