What the fisherman had left in Mile End (4)
I believe the answer is:
line
'what the fisherman' is the definition.
I can't judge whether this definition defines the answer.
'left in mile end' is the wordplay.
'left' becomes 'l' (common abbreviation).
'end' says to take the final letters.
The final letter of 'mile' is 'e'.
'l'+'in'+'e'='LINE'
'had' acts as a link.
Can you help me to learn more?
(Other definitions for line that I've seen before include "Wrinkle; row" , "Fill" , "Policy - file" , "Phone connection; queue" , "short note" .)