A line slipped into big report for added weight (7)
I believe the answer is:
ballast
'added weight' is the definition.
(I've seen this in another clue)
'a line slipped into big report' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'slipped into' indicates putting letters inside.
'big report' becomes 'blast' (report can mean the noise from an explosion).
'a'+'l'='al'
'al' placed within 'blast' is 'BALLAST'.
'for' is the link.
(Other definitions for ballast that I've seen before include "Material put into a ship to improve its stability" , "Material for railway bed" , "Material used to stabilize a ship" , "Stabilising material" , "Stabilising weight (naut.)" .)