Preferable, in fact, to be studying locally (4)
I believe the answer is:
best
'preferable in fact' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'be studying locally' is the wordplay.
'studying locally' becomes 'st' (I can't justify this - if you can you should believe this answer much more).
'be'+'st'='BEST'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for best that I've seen before include "Worst -- as good as can be" , "at 16 down [MAN UNITED] ?" , "Superlatively good" , "Cream of the crop" , "Most excellent - late football star" .)