The most common and most likely answer to this crossword clue is the 4 letter word PONS
P
O
N
S
Buddy explains!
The straight part
We know that "What’ll form link, physically" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what’ll form link, physically = pons.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 PONS mean?
PONS noun- A bridge-like tissue connecting two parts of an organ.
- A band of nerve fibres, from the Latin term pōns Varoliī, within the brain stem.