### Clue: Colonel's first to board plane, after troops decline (6)
### Answer: REJECT 
 ### Explanation: Definition: decline
 [**1st letter of**(…'s first) "Colonel" **contained in**(to board) JET(an airplane powered by, well, jet engines)] **placed after**(after) RE(abbrev. for the Royal Engineers, troops in the British Army).
Defn: …/to not accept.