Ruler in mood beheaded soldiers (7)
I believe the answer is:
emperor
'ruler' is the definition.
(emperor is a kind of ruler)
'mood beheaded soldiers' is the wordplay.
'mood' becomes 'temper' (temper is one's mood or state of mind).
'beheaded' means to remove the first letter (the head of the word is removed).
'soldiers' becomes 'OR' (Other Ranks).
'temper' with its first letter taken off is 'emper'.
'emper'+'or'='EMPEROR'
'in' is the link.
(Other definitions for emperor that I've seen before include "A moth; a penguin" , "Large richly coloured butterfly" , "Largest type of penguin" , "Title of sovereignty" , "Ruler; butterfly" .)