Experiment on patient may set a precedent (4,4)
I believe the answer is:
test case
'a precedent' is the definition.
(I've seen this before)
'experiment on patient' is the wordplay.
'experiment' becomes 'test' (test is a kind of experiment).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'patient' becomes 'case' (synonyms).
'test'+'case'='TEST-CASE'
'may set' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for test case that I've seen before include "Lawsuit whose decision will form precedent" , "Legal action setting a precedent" , "Legal precedent setter" , "Legal precedent-setter" , "that could set a precedent" .)