Publication on web is source of much attraction (9)
I believe the answer is:
magnetism
'attraction' is the definition.
(I know that magnetism is a type of attraction)
'publication on web is source of much' is the wordplay.
'publication' becomes 'mag' (abbreviation for magazine).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'web' becomes 'net' (synonyms).
'source of' indicates taking the first letters (the start - or source - of the word).
The initial letter of 'much' is 'm'.
'mag'+'net'+'is'+'m'='MAGNETISM'
(Other definitions for magnetism that I've seen before include "Drawing power" , "Attractive force" , "Attraction" , "Charisma" .)