Roman emperor in a hard form (7)
I believe the answer is:
hadrian
'roman emperor' is the definition.
(I've seen this before)
'in a hard form' is the wordplay.
'form' indicates an anagram (another form of the letters).
'in'+'a'+'hard'='inahard'
'inahard' is an anagram of 'HADRIAN'.
(Other definitions for hadrian that I've seen before include "His wall crosses northern Britain" , "Roman emperor who built the British wall" , "Wall-building Roman emperor" , "Roman emperor who visited Britain" , "Imperial wall-builder" .)