The most common and most likely answer to this crossword clue is the 3 letter word GIS
I've seen this crossword clue in The Guardian
G
I
S
Buddy explains!
The straight part
We know that "Grunts" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case grunts = gis.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 GIS mean?
GIS noun- The physical and behavioural characteristics of a bird that enable it to be immediately recognised by an experienced birder as a certain type of bird, especially to family or genus level.
- Male ejaculate; sperm, semen.