Weak shell of elegant mollusc (6)
I believe the answer is:
limpet
'mollusc' is the definition.
(I know that Limpet is a type of sea snail)
'weak shell of elegant' is the wordplay.
'weak' becomes 'limp' (I've seen this before).
'shell of' means to remove the middle letters.
'elegant' with its middle taken out is 'et'.
'limp'+'et'='LIMPET'
(Other definitions for limpet that I've seen before include "Marine creature clinging to a rock" , "Rock lover" , "he's not easily got rid of" , "clingy type" , "Shellfish; type of mine" .)