Troop formation has no shelter retreating, Switzerland admitted (7)
I believe the answer is:
echelon
'troop formation' is the definition.
(I've seen this in another clue)
'no shelter retreating switzerland admitted' is the wordplay.
'shelter' becomes 'lee' (as in lee side, the side sheltered from wind).
'retreating' shows that the letters should be reversed in order.
'switzerland' becomes 'ch' (short for Confoederatio Helvetica, its Latin name).
'admitted' indicates putting letters inside (inserted letters are let in or admitted).
'no'+'lee'='nolee'
'nolee' written backwards gives 'eelon'.
'eelon' enclosing 'ch' is 'ECHELON'.
'has' acts as a link.
(Other definitions for echelon that I've seen before include "Grade or level" , "Military row formation" , "Degree" , "Staggering display of military" , "Bishops, say" .)