Colonel's first to board plane, after troops decline (6)
I believe the answer is:
reject
'decline' is the definition.
(rejecting is a kind of declining)
'colonel's first to board plane after troops' is the wordplay.
'first' suggests taking the first letters.
'to board' means one lot of letters goes inside another.
'plane' becomes 'jet' (jet is a kind of plane).
'after' means one lot of letters go next to another.
'troops' becomes 're' (I've seen this in other clues**).
The first letter of 'colonels' is 'c'.
'c' put within 'jet' is 'ject'.
'ject' after 're' is 'REJECT'.
(Other definitions for reject that I've seen before include "substandard article" , "Refuse to accept or acknowledge" , "Decline" , "Refuse to accept, spurn" , "Turn (offer) down" .)