Nintendo ports a game over partially — it moves slowly (9)
I believe the answer is:
gastropod
'it moves slowly' is the definition.
(gastropods include snails, slugs etc)
'nintendo ports a game over partially' is the wordplay.
'partially' indicates the answer is hidden within the clue.
'over' shows that the letters should be reversed in order.
'doportsag' can be found hidden inside 'nintendo ports a game'.
'doportsag' written backwards gives 'GASTROPOD'.
(Other definitions for gastropod that I've seen before include "Could be snail" , "sluggish type" , "Escargot" , "Shellfish" , "Eg, a snail or slug" .)