Marine arthropod man on board originally rescued in taxi (4,4)
I believe the answer is:
king crab
'marine arthropod' is the definition.
'man on board originally rescued in taxi' is the wordplay.
'man on board' becomes 'king' (chess).
'originally' indicates taking the first letters.
'in' indicates putting letters inside.
'taxi' becomes 'cab' ('cab' can be a synonym of 'taxi').
The initial letter of 'rescued' is 'r'.
'r' inserted into 'cab' is 'crab'.
'king'+'crab'='KING-CRAB'
(Other definitions for king crab that I've seen before include "seafood restaurant order" , "Large crustacean" , "Marine creature" .)