Whales or crabs culled at intervals (5)
I believe the answer is:
'whales' is the definition.
(orca is a kind of whale)
'or crabs culled at intervals' is the wordplay.
I cannot quite understand how this works, but
'or' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for orcas that I've seen before include "Killers" , "Killer whales" .)