The most common and most likely answer to this crossword clue is the 3 letter word EBB
I've seen this crossword clue in Financial Times
E
B
B
More clues leading to the result EBB
- Finally lose black bishop, fall away
- Energy brilliant, but starts to decline
- English bishops decline
- English bishops become less intense
- h-honeycomb maker briefly turns to? Not to wax
- Withdraw European books
- English books fall back
- Finally wave bye bye and go out
- Decline eastern bishop twice
- Retreat from eager beaver’s bossiness at first
Buddy explains!

The straight part
We know that "Decline" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case decline = ebb.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 EBB mean?
EBB noun- The receding movement of the tide.
- A gradual decline.
- A low state; a state of depression.
- A European bunting, the corn bunting (Emberiza calandra, syns. Emberiza miliaria, Milaria calandra).
- To flow back or recede
- To fall away or decline
- To fish with stakes and nets that serve to prevent the fish from getting back into the sea with the ebb
- To cause to flow back.
- Low, shallow