Orlando ports agree to hold up mollusc (9)
I believe the answer is:
gastropod
'mollusc' is the definition.
(I know that gastropod is a type of mollusc)
'orlando ports agree to hold up' is the wordplay.
'to hold' indicates the answer is hidden within the clue (I've seen 'holding' mean this).
'up' is a reversal indicator (in down clue: letters go upwards).
'doportsag' can be found hidden inside 'orlando ports agree'.
'doportsag' in reverse letter order is 'GASTROPOD'.
(Other definitions for gastropod that I've seen before include "Shellfish" , "slug, perhaps" , "Mollusc such as snail or limpet" , "Could be snail" , "Escargot" .)