In which the king crab set up residence? (4,6)
I believe the answer is:
sand castle
'in which the king crab set' is the definition.
The definition suggests a singular noun which matches the answer.
'up residence?' is the wordplay.
'up' becomes 'sand' (I am not sure about this - if you are sure you should believe this answer much more).
'residence?' becomes 'castle' (castle is a kind of residence).
'sand'+'castle'='SAND CASTLE'
Can you help me to learn more?
(Other definitions for sand castle that I've seen before include "Beach construction" , "Fortress constructed on the beach" , "Beach fortification?" , "Seaside construction" .)