He attacked the Romans but turned tail at first (6)
I believe the answer is:
attila
'he attacked the romans' is the definition.
(I've seen this before)
'turned tail at first' is the wordplay.
'turned' indicates anagramming the letters.
'first' says to put letters next to each other.
'tail' anagrammed gives 'tila'.
'tila' after 'at' is 'ATTILA'.
'but' acts as a link.
(Other definitions for attila that I've seen before include "Fifth century king of the Huns" , "King of the Huns, invader of the Roman Empire" , "Fifth century Hunnish leader" , "conquered Europe" , "The greatest Hun leader" .)