Ache to join key troops in the near future (7)
I believe the answer is:
erelong
'in the near future' is the definition.
(erelong means soon)
'ache to join key troops' is the wordplay.
'ache' becomes 'long' (synonyms).
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'join key' becomes 'e' (I can't justify this - if you can you should give a lot more credence to this answer).
'troops' becomes 'RE' (Royal Engineers army regiment).
'e'+'re'='ere'
'long' put after 'ere' is 'ERELONG'.
(Other definitions for erelong that I've seen before include "Soon (archaic)" , "Soon (poetic)" , "Soon (arch.)" , "Soon (poetically)" .)