Long fish delivered in Billingsgate (4)
I believe the answer is:
ache
'long' is the definition.
(to long for something is to ache for it)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for ache that I've seen before include "Similar" , "Dull continuous pain" , "anguish" , "Hurt; yearn" , "Longing" .)