Chucks the fish aboard (6)
I believe the answer is:
slings
'chucks the fish aboard' is the definition.
I can't judge whether this defines the answer.
'chucks the fish aboard' is the wordplay.
'chucks' becomes 'ss' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'the fish' becomes 'ling' (ling is a kind of fish).
'aboard' means one lot of letters goes inside another.
'ss' enclosing 'ling' is 'SLINGS'.
Can you help me to learn more?
(Other definitions for slings that I've seen before include "Arm bandages; drinks" , "'''... to suffer the ... and arrows of outrageous fortune'' (Hamlet) (6)'" , "Supporters" , "Looped bandages for limb support or missile throwers" , "Catapults or arm supports" .)