Helen & Co changed formation (7)
I believe the answer is:
echelon
'formation' is the definition.
(I've seen this before)
'helen co changed' is the wordplay.
'changed' indicates anagramming the letters.
'helen'+'co'='helenco'
'helenco' anagrammed gives 'ECHELON'.
(Other definitions for echelon that I've seen before include "Degree" , "position in society" , "Staggering display of military" , "Grade or level" , "Ladder formation" .)