Write about love - a newspaper man's first - expressed in legal document (8)
I believe the answer is:
subpoena
'legal document' is the definition.
(type of legal document)
'write about love a newspaper man's first' is the wordplay.
'write' becomes 'pen' (synonyms).
'about' means one lot of letters goes inside another.
'love' becomes 'o' (love means zero in tennis).
'newspaper man' becomes 'sub' (short for subeditor).
'first' is a charade indicator (letters next to each other).
'pen' enclosing 'o' is 'poen'.
'poen'+'a'='poena'
'poena' after 'sub' is 'SUBPOENA'.
'expressed in' acts as a link.
(Other definitions for subpoena that I've seen before include "Court order to attend as a witness" , "Summon to court" , "Writ compelling attendance in court" , "Order to appear in court" , "Writ ordering court apperarance about bean soup" .)