Following suit, one swots up on it (4,4)
I believe the answer is:
test case
'following' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
'suit one swots up on it' is the wordplay.
'suit' becomes 'test suit' (test suit is a kind of suit).
'one swots up' becomes 'case' (I can't explain this - if you can you should believe this answer much more).
'on it' is a charade indicator (letters next to each other).
'test'+'case'='TEST-CASE'
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 where the outcome will serve as a precedent" , "Legal action setting a precedent" , "Legal precedent setter" , "Possible legal precedent" .)