Perhaps Vespasian's mood heading off with enlisted men (7)
I believe the answer is:
emperor
'perhaps vespasian's' is the definition.
(Vespasian was a Roman emperor)
'mood heading off with enlisted men' is the wordplay.
'mood' becomes 'temper' (synonyms).
'heading off' means to remove the first letter.
'with' says to put letters next to each other.
'enlisted men' becomes 'OR' (military abbreviation for Other Ranks).
'temper' with its first letter removed is 'emper'.
'emper'+'or'='EMPEROR'
(Other definitions for emperor that I've seen before include "A moth; a penguin" , "Monarch" , "chap such as Charlemagne" , "Large richly-coloured butterfly" , "Kaiser" .)