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' says the letters should be written backwards.
'switzerland' becomes 'ch' (short for Confoederatio Helvetica, its Latin name).
'admitted' is an insertion indicator (inserted letters get admitted).
'no'+'lee'='nolee'
'nolee' in reverse letter order is 'eelon'.
'eelon' going around 'ch' is 'ECHELON'.
'has' is the link.
(Other definitions for echelon that I've seen before include "Grade or level" , "Military row formation" , "Degree" , "Staggering display of military" , "Bishops, say" .)