The most common and most likely answer to this crossword clue is the 6 letter word DORSAL
I've seen this crossword clue in Financial Times
D
O
R
S
A
L
Buddy explains!
The straight part
We know that "Relating to the back part of pilchard or salmon" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case relating to the back part of pilchard or salmon = dorsal.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does DORSAL mean?
DORSAL noun- A hanging, usually of rich stuff, at the back of a throne, altar, etc.
- With respect to, or concerning the side in which the backbone is located, or the analogous side of an invertebrate.
- (of a knife) Having only one sharp side.
- Relating to the top surface of the foot or hand.
- (of a sound) Produced using the dorsum of the tongue.
- Relating to the surface naturally inferior, as of a leaf.
- Relating to the surface naturally superior, as of a creeping hepatic moss.