Crustacean caught by Scotsman on net (4,4)
I believe the answer is:
land crab
'crustacean' is the definition.
(I've seen this before)
'caught by scotsman on net' is the wordplay.
'caught by' becomes 'c' (cricket abbreviation. I am not sure about the 'by' bit.).
'scotsman' becomes 'Rab' (Rab is a Scottish name).
'on' is a charade indicator (letters next to each other).
'net' becomes 'land' (both can mean to secure or obtain).
'c'+'rab'='crab'
'crab' after 'land' is 'LAND-CRAB'.
(Other definitions for land crab that I've seen before include "Burrowing crustacean" , "Mainly terrestrial crustacean" , "Violent criminal" , "one with nippers" .)