Intelligence about Republican leads to legal document (4)
I believe the answer is:
writ
'legal document' is the definition.
(writ is a kind of legal document)
'intelligence about republican leads' is the wordplay.
'intelligence' becomes 'wit' (wit is a kind of intelligence**).
'about' is an insertion indicator.
'leads' suggests taking the first letters.
The initial letter of 'republican' is 'r'.
'wit' going around 'r' is 'WRIT'.
'to' acts as a link.
(Other definitions for writ that I've seen before include "Court instrument" , "Warrant, court order" , "court paper" , "Legal injunction" , "Legal document of command" .)