Schoolboy is son missed by Baltic citizen (7)
I believe the answer is:
etonian
'schoolboy' is the definition.
(I've seen this before)
'son missed by baltic citizen' is the wordplay.
'son' becomes 's'.
'missed by' indicates named letters should be taken away.
'baltic citizen' becomes 'estonian'.
'estonian' with 's' taken away is 'ETONIAN'.
'is' acts as a link.
(Other definitions for etonian that I've seen before include "student in establishment?" , "Public school boy" , "Public schoolboy" , "Alumnus of famous English school" , "Relating to one particular school" .)