Fish swallowing end of angler's line (4)
I believe the answer is:
cord
'line' is the definition.
(I know that cord is a type of line)
'fish swallowing end of angler's' is the wordplay.
'fish' becomes 'cod' (type of fish).
'swallowing' means one lot of letters goes inside another.
'end of' suggests the final letters.
The final letter of 'angler' is 'r'.
'cod' placed around 'r' is 'CORD'.
(Other definitions for cord that I've seen before include "A measure of cut wood" , "Small rope or thick string" , "Small rope; flex" , "Rope; fabric" , "Thin rope" .)