Whinge from signalmen who are caught in the sea by one of those involved in a cast (6)
I believe the answer is:
Here is my best explanation:
'whinge from signalmen who are caught in the sea by one of those involved in a cast' is the definition.
The definition suggests a singular noun which matches the answer.
'whinge from signalmen who are caught in' is the subsidiary indication.
'from' suggests deleting specific letters.
'are caught in' means one lot of letters goes inside another.
'signalmen' placed around 'who' is 'signalmewhon'.
'signalmewhon' with 'whinge' taken away is 'salmon'.
(Other definitions for salmon that I've seen before include "See 3", "Legendary fish of knowledge", "Pink fish", "Freshwater fish; colour", "Fish; pink colour", "Edible fish", "Pink shade?", "(Now mainly farmed) fish", "Food fish; pink shade" and "'No alms, oddly, for this fish (6)'".)