Nominally a Muscovite? Not Russ! (3)
I believe the answer is:
ian
'nominally a' is the definition.
I can't tell whether this defines the answer.
'muscovite? not russ' is the wordplay.
'muscovite?' becomes 'Russian' (muscovite is a kind of Russian).
'not' is a deletion indicator.
'russian' with 'russ' taken out is 'IAN'.
Can you help me to learn more?
(Other definitions for ian that I've seen before include "Boy's name (Scottish?)" , "Scotsman?" , "The familiar Dr Paisley" , "The familar Dr Paisley" , "Male" .)