Ruler abroad forbidding endless revel (7)
I believe the answer is:
roister
'revel' is the definition.
(both can mean to party)
'ruler abroad forbidding endless' is the wordplay.
'ruler abroad' becomes 'roi' ('king' in French).
'forbidding' becomes 'stern' (synonyms).
'endless' means to remove the last letter.
'stern' with its final letter removed is 'ster'.
'roi'+'ster'='ROISTER'
(Other definitions for roister that I've seen before include "Carouse" , "bluster" , "Revel noisily, swagger" , "Be boisterous" , "Rioters (anag.)" .)