Heartily catch fish in stream (4)
I believe the answer is:
tide
'stream' is the definition.
(thesaurus)
'heartily catch fish' is the wordplay.
'heartily' says to take the centre.
'fish' becomes 'ide' (I've seen this before).
The central letter of 'catch' is 't'.
't'+'ide'='TIDE'
'in' is the link.
(Other definitions for tide that I've seen before include "Ebber and flower?" , "Moon-driven sea movement" , "turning point" , "Ebb and flow of the sea" , "Time and it wait for no man" .)