Team's leader assists securing a mountain range (3,4)
I believe the answer is:
the alps
'mountain range' is the definition.
(the Alps is a kind of mountain range)
'team's leader assists securing a' is the wordplay.
'leader' suggests taking the first letters.
'assists' becomes 'helps' ('help' can be a synonym of 'assist').
'securing' means one lot of letters goes inside another.
The first letter of 'teams' is 't'.
'helps' going around 'a' is 'healps'.
't'+'healps'='THE ALPS'
(Other definitions for the alps that I've seen before include "heights" , "Range" , "natural barrier" .)