Third of total fish within boundary caught (9)
I believe the answer is:
entangled
'caught' is the definition.
(I've seen this before)
'third of total fish within boundary' is the wordplay.
'third of total' becomes 't' (grd letter of 'total').
'fish' becomes 'angle' (angle can mean to fish).
'within' means one lot of letters goes inside another.
'boundary' becomes 'end' (end is a kind of boundary).
't'+'angle'='tangle'
'tangle' going into 'end' is 'ENTANGLED'.
(Other definitions for entangled that I've seen before include "caught" , "Involved in complications" , "snarled up" , "Confusedly twisted" , "Intertwined; entrapped" .)