The most common and most likely answer to this crossword clue is the 4 letter word ROES
I've seen this crossword clue in The Daily Telegraph
R
O
E
S
More clues leading to the result ROES
Buddy explains!

The straight part
We know that "Deer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case deer = roes.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 ROES mean?
ROES noun- The eggs of fish.
- The sperm of certain fish.
- The ovaries of certain crustaceans.
- Short for roe deer.
- A mottled appearance of light and shade in wood, especially in mahogany.