It means something when you take on a novice (6)
I believe the answer is:
signal
'it means something' is the definition.
The definition suggests a singular noun which matches the answer.
'you take on a novice' is the wordplay.
'you take' becomes 'sign' (I can't explain this - if you can you should believe this answer much more).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'novice' becomes 'L' (as in a learner driver with L-plates on their car).
'sign'+'a'+'l'='SIGNAL'
'when' acts as a link.
Can you help me to learn more?
(Other definitions for signal that I've seen before include "Non-verbal action with an encoded message" , "Noteworthy" , "Gesture giving information or command" , "Gesture; notable" , "Extraordinary" .)