In with a thousand, working with another
I believe the answer is:
among
'in with' is the definition.
(I've seen this in another clue)
'a thousand working with another' is the wordplay.
'thousand' becomes 'm' (synonyms).
'working' becomes 'on' (on can mean working or activated).
'with another' becomes 'g' (I can't justify this - if you can you should give a lot more credence to this answer).
'a'+'m'+'on'+'g'='AMONG'
Can you help me to learn more?
(Other definitions for among that I've seen before include "Go man! (anag.)" , "In the middle of, say a crowd" , "Included with - inside" , "Together with" , "Amidst" .)