Back company involved in project (6)
I believe the answer is:
second
'back' is the definition.
(synonyms)
'company involved in project' is the wordplay.
'company' becomes 'co' (abbreviation).
'involved' is an insertion indicator.
'in project' becomes 'send' (I can't explain this - if you can you should believe this answer much more).
'co' inserted inside 'send' is 'SECOND'.
Can you help me to learn more?
(Other definitions for second that I've seen before include "Small fraction of a day" , "Short time - back" , "Flash" , "Moment; musical interval" , "Lower < back" .)