The charm of a soldier in a raincoat (5)
I believe the answer is:
magic
'the charm' is the definition.
(thesaurus)
'a soldier in a raincoat' is the wordplay.
'a soldier' becomes 'GI' (term for US soldier).
'in' is an insertion indicator.
'a raincoat' becomes 'mac' (macintosh is a type of raincoat).
'gi' put within 'mac' is 'MAGIC'.
'of' is the link.
(Other definitions for magic that I've seen before include "Enchantment" , "Fungus causing trips?" , "Enchanting quality" , "Superb" , "Witchcraft" .)