An injection, yes, with British capital (3)
I believe the answer is:
'an injection' is the definition.
(I've seen this before)
'yes with british capital' is the wordplay.
'yes with' becomes 'ja' (I can't explain this - if you can you should give a lot more credence to this answer).
'capital' says to take the initial letters.
The first letter of 'british' is 'b'.
Can you help me to learn more?
(Other definitions for jab that I've seen before include "Injection" , "A short stiff punch" , "Poke roughly or inject" , "Prod" , "Short stiff blow or thrust" .)