I am confident of these letters a,e
8d which gives the o clue is :
Canadian's 18's wonderful real ale (5,6)
I have Great Porter
16d (giving the I)
....where Romans were in position in the first place (2,4)
have in situ
Looks promising (although I can't parse it all yet)
We may need to verify the O from the clue Canadian's 18's wonderful real ale (5,6) . What was the answer to 18?