Roman emperor crushed a Gallic uprising initially (8)
I believe the answer is:
caligula
'roman emperor' is the definition.
(I've seen this before)
'crushed a gallic uprising initially' is the wordplay.
'crushed' indicates anagramming the letters.
'initially' indicates taking the first letters.
The initial letter of 'uprising' is 'u'.
'a'+'gallic'+'u'='agallicu'
'agallicu' is an anagram of 'CALIGULA'.
(Other definitions for caligula that I've seen before include "imperial figure" , "Third Roman emperor" , "Emperor who threatened to make his horse a Consul" , "The Roman emperor said to have made his horse a consul" , "Successor to Tiberius" .)