Pack makes fellow sick (4)
I believe the answer is:
fill
'pack' is the definition.
(I know that pack is a kind of fill)
'fellow sick' is the wordplay.
'fellow' becomes 'f' (abbreviation - of a society etc.).
'sick' becomes 'ill' (similar in meaning).
'f'+'ill'='FILL'
'makes' acts as a link.
(Other definitions for fill that I've seen before include "Block up (a hole, etc)" , "Become satiated" , "Occupy the whole of" , "Repair tooth" , "Completely occupy" .)