Imagine French are enthralled by Augustus (5)
I believe the answer is:
guess
'imagine' is the definition.
(I know that imagine can be written as guess)
'french are enthralled by augustus' is the wordplay.
'french are' becomes 'es' ('are' in French).
'enthralled by' means one lot of letters goes inside another ('enthral' can mean 'capture').
'augustus' becomes 'gus' (I am not sure about this - if you are sure you should believe this answer much more).
'es' inserted inside 'gus' is 'GUESS'.
Can you help me to learn more?
(Other definitions for guess that I've seen before include "Estimate on the basis of little information" , "shot?" , "Reckon" , "the Americans believe it" , "Speculate" .)