Cavalryman to free soldiers up (5)
I believe the answer is:
rider
'cavalryman' is the definition.
(thesaurus)
'free soldiers up' is the wordplay.
'free' becomes 'rid' (to be free of something is to be rid of it).
'soldiers' becomes 're' (Royal Engineers).
'up' says the letters should be written backwards (in down clue: letters go upwards).
're' in reverse letter order is 'er'.
'rid'+'er'='RIDER'
'to' acts as a link.
(Other definitions for rider that I've seen before include "qualification" , "in the dressing room before the show?" , "Motorcyclist" , "Equestrian - clause" , "it may be added" .)