Troop’s leader supports capturing a mountain range (3,4)
I believe the answer is:
the alps
'mountain range' is the definition.
(I know this)
'troop's leader supports capturing a' is the wordplay.
'leader' suggests taking the first letters.
'supports' becomes 'helps' (helping is a kind of supporting).
'capturing' means one lot of letters goes inside another.
The initial letter of 'troops' is 't'.
'helps' placed around 'a' is 'healps'.
't'+'healps'='THE ALPS'
(Other definitions for the alps that I've seen before include "heights" , "natural barrier" , "Range" .)