Start removing number of Romans from testudo in retreat (6)
I believe the answer is:
outset
'start' is the definition.
(synonyms)
'removing number of romans from testudo in retreat' is the wordplay.
I cannot really understand how this works, but
'number' could be 'set' (I have seen 'number of games' mean 'set' so perhaps 'number' could also mean 'set') and 'set' is present in the answer.
'of' could be 'o' and 'o' is found within the answer.
The remaining letters 'ut' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for outset that I've seen before include "origin" , "Starting point" , "Opening stage" , "Fruit" , "Beginning, start" .)