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 understand how this works, but
'modern' could be 'in' (I've seen this in other clues) and 'in' is located in the answer.
The remaining letters 'len' 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 lenin that I've seen before include "Revolutionary leader" , "V. I. -, Soviet politician and revolutionary" , "Revolutionary man" , "Vladimir Ilich Ulyanov" , "Russian revolutionary" .)