Correspondence, some enclosing a journal (7)
I believe the answer is:
analogy
'correspondence' is the definition.
(I've seen this before)
'some enclosing a journal' is the wordplay.
'some' becomes 'any' (similar in meaning).
'enclosing' means one lot of letters goes inside another.
'journal' becomes 'log' (log can mean a journal or diary).
'a'+'log'='alog'
'any' enclosing 'alog' is 'ANALOGY'.
(Other definitions for analogy that I've seen before include "A clarifying comparison" , "Correspondence, resemblance" , "Instructive comparison between two things" , "Partial likeness between two things being compared" , "A comparison or parallel" .)