Fish bone at last reveals secret message (4)
I believe the answer is:
code
'secret message' is the definition.
(I've seen this in another clue)
'fish bone at last' is the wordplay.
'fish' becomes 'cod' (type of fish).
'at last' says to take the final letters.
The last letter of 'bone' is 'e'.
'cod'+'e'='CODE'
'reveals' is the link.
(Other definitions for code that I've seen before include "Established standards" , "Kind of encryption designed to disguise content" , "conventions" , "Digest of laws" , "Ethical standards" .)