That's right, six months at Strangeways, initially! (3)
I believe the answer is:
yes
'that's right' is the definition.
(right can mean yes or okay)
'six months at strangeways initially' is the wordplay.
'six months at' becomes 'ye' (I can't explain this - if you can you should believe this answer much more).
'initially' indicates taking the first letters.
The first letter of 'strangeways' is 's'.
'ye'+'s'='YES'
Can you help me to learn more?
(Other definitions for yes that I've seen before include "Affirmative answer" , "3 across" , "Certainly!" , "Yea" , "OK" .)