Impinge on fish behind enclosure (8)
I believe the answer is:
encroach
'impinge' is the definition.
('encroach' can be a synonym of 'impinge')
'fish behind enclosure' is the wordplay.
'fish' becomes 'roach' (roach is a kind of fish**).
'behind' is a charade indicator (letters next to each other).
'enclosure' becomes 'enc'.
'roach' put after 'enc' is 'ENCROACH'.
'on' acts as a link.
(Other definitions for encroach that I've seen before include "Advance gradually (beyond expected limits)" , "Advance beyond proper limit" , "Invade" , "'Trespass, infringe (8)'" , "Advance beyond the usual limit" .)