Gathered a great deal by journalist (7)
I believe the answer is:
amassed
'gathered' is the definition.
('amass' can be a synonym of 'gather')
'a great deal by journalist' is the wordplay.
'great deal' becomes 'mass' (synonyms).
'by' means one lot of letters go next to another.
'journalist' becomes 'ed' (short for editor).
'a'+'mass'+'ed'='AMASSED'
(Other definitions for amassed that I've seen before include "Gathered together" , "Mad seas (anag.)" , "Collected" , "Accumulated a great quantity" , "Got up" .)