Experienced upset spearing rather dainty molluscs
I believe the answer is:
cuttlefish
'experienced upset' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for cuttlefish that I've seen before include "Squid-like marine mollusc" , "An inshore cephalopod" , "Mollusc, source of sepia" , "Squid's cousin" , "Mollusc - leftish cut (anag)" .)