Horse soldiers brought back in but multiplied (12)
I believe the answer is:
thoroughbred
'horse' is the definition.
(thesaurus)
'soldiers brought back in but multiplied' is the wordplay.
'soldiers' becomes 'or' (Other Rank).
'brought back' shows that the letters should be reversed in order.
'in' indicates putting letters inside.
'but' becomes 'though' (I've seen this before**).
'multiplied' becomes 'bred' (breeding is a kind of multiplying).
'or' written backwards gives 'ro'.
'ro' going within 'though' is 'thorough'.
'thorough'+'bred'='THOROUGHBRED'
(Other definitions for thoroughbred that I've seen before include "Pure-blooded racehorse" , "A horse of pure lineage" , "Of outstanding quality" , "of highest quality" , "One of pure pedigree" .)