I will follow fellow and pack (4)
I believe the answer is:
fill
'pack' is the definition.
(I know that pack is a kind of fill)
'i will follow fellow' is the wordplay.
'i will' becomes 'I'll' (abbreviation).
'follow' says to put letters next to each other.
'fellow' becomes 'f' (abbreviation - of a society etc.).
'ill' after 'f' is 'FILL'.
'and' acts as a link.
(Other definitions for fill that I've seen before include "` `Work expands to .... the time available' '" , "Satisfy (a need)" , "Plug (a gap)" , "Completely occupy" , "Top up" .)