Mollusc that has one looking amazed? (5)
I believe the answer is:
gaper
This clue is a double definition.
'mollusc' is the first definition.
(I've seen this before)
'one looking amazed?' is the second definition.
(someone agape in shock)
'that has' is the link.
(Other definitions for gaper that I've seen before include "who yawns?" , "one visibly amazed" , "one staring in wonder" , "Fish; burrowing mollusc" , "Anglerfish" .)