Ruler, in rage, decapitated soldiers (7)
I believe the answer is:
emperor
'ruler' is the definition.
(emperor is a kind of ruler)
'rage decapitated soldiers' is the wordplay.
'rage' becomes 'temper' (temper means anger or rage).
'decapitated' means to remove the first letter.
'soldiers' becomes 'OR' (Other Ranks).
'temper' with its first letter taken away is 'emper'.
'emper'+'or'='EMPEROR'
'in' is the link.
(Other definitions for emperor that I've seen before include "Sovereign and richly-colored butterfly" , "He rules over several states" , "Penguin of largest known species - a great ruler" , "chap such as Charlemagne" , "Beethoven's Piano Concerto No. 5 in E-flat major" .)