Fish that’s stuffed with diamonds (3)
I believe the answer is:
ide
'fish' is the definition.
(I've seen this before)
'that's stuffed with diamonds' is the wordplay.
'that's' becomes 'ie'.
'stuffed with' is an insertion indicator (some letters get stuffed inside).
'diamonds' becomes 'd' (abbreviation for the card suit diamonds).
'ie' enclosing 'd' is 'IDE'.
(Other definitions for ide that I've seen before include "Fish of the carp family" , "Fish, the orfe" , "Silver orfe" .)