Sign I’d led men astray with just one purpose (6-6)
I believe the answer is:
single-minded
'with just one purpose' is the definition.
'sign i'd led men astray' is the wordplay.
'astray' indicates an anagram.
'sign'+'id'+'led'+'men'='signidledmen'
'signidledmen' is an anagram of 'SINGLE-MINDED'.
(Other definitions for single-minded that I've seen before include "Dedicated to one outcome" , "Concentrating purposefully" , "Concentrating on only one aim" , "Bent upon one sole purpose" , "Not distracted from chosen purpose" .)