He helped rename modern St. Petersburg (5)
I believe the answer is:
lenin
'he helped rename' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'modern st petersburg' is the wordplay.
I cannot quite see how this works, but
'modern' could be 'in' (I've seen this in other clues) and 'in' is found within the answer.
The remaining letters 'len' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for lenin that I've seen before include "Revolutionary leader" , "V. I. -, Soviet politician and revolutionary" , "Revolutionary man" , "Vladimir Ilich Ulyanov" , "Russian revolutionary" .)