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' suggests the final letters.
The last 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" .)