Perhaps Nicholas or Alexander's son nursed by Jack (4)
I believe the answer is:
tsar
'perhaps nicholas or alexander's' is the definition.
I can't tell whether this definition defines the answer.
'son nursed by jack' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'nursed by' means one lot of letters goes inside another.
'jack' becomes 'tar' (both are informal terms for sailor).
's' going inside 'tar' is 'TSAR'.
Can you help me to learn more?
(Other definitions for tsar that I've seen before include "Title given to the kings of Bulgaria" , "Rats to the Russian emperor" , "Nicholas II was the last" , "Autocrat" , "specific authority" .)