Strike is appropriate as one goes for zero (6)
I believe the answer is:
picket
'zero' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'strike is appropriate as one goes' is the wordplay.
'strike' becomes 'pick' (picking is a kind of striking).
'is appropriate as one goes' becomes 'et' (I can't explain this - if you can you should give a lot more credence to this answer).
'pick'+'et'='PICKET'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for picket that I've seen before include "Group outside workplace encouraging strike action" , "Trade union protestor(s) outside a place of work" , "Striker's line outside workplace" , "Pale" , "Small patrol" .)