Ruler in mood beheaded other ranks (7)
I believe the answer is:
emperor
'ruler' is the definition.
(I've seen this before)
'mood beheaded other ranks' is the wordplay.
'mood' becomes 'temper' ('temper' can be a synonym of 'mood').
'beheaded' means to remove the first letter.
'other ranks' becomes 'or' (military abbreviation).
'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 "Japanese version of the king" , "Beethoven's Piano Concerto No. 5 in E-flat major" , "A moth; a penguin" , "Absolute ruler" , "Male ruler of a group of countries" .)