### Clue: Invitee and one chum give a rough idea (10)
### Answer: GUESTIMATE 
 ### Explanation: Definition: a rough idea
  GUEST(*invitee*) +(*and*) I(=*one*) MATE(*chum*).
Defn:  A portmanteau word made up of "guess" and "estimate", in other words, not much better than a guess.  Though used quite often for years when I was working, it looks as if it got into Collins only in 2009.