Take too long to capture old soldier (7)
I believe the answer is:
dragoon
'soldier' is the definition.
(I know that dragoon is a type of Trooper)
'take too long to capture old' is the wordplay.
'take too long' becomes 'drag on' (to drag on is to go on too long).
'to capture' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'dragon' placed around 'o' is 'DRAGOON'.
(Other definitions for dragoon that I've seen before include "Former heavily armed cavalryman" , "Harass" , "Cavalry soldier" , "Force" , "Cavalryman; coerce (into doing something)" .)