Foolish person falling short as a friend (4)
I believe the answer is:
chum
'a friend' is the definition.
(chum is a kind of friend)
'foolish person falling short' is the wordplay.
'foolish person' becomes 'chump' (I've seen this before).
'falling short' means to remove the last letter.
'chump' with its last letter removed is 'CHUM'.
'as' is the link.
(Other definitions for chum that I've seen before include "Good friend" , "Pal, buddy" , "Pal, mate" , "Close friend (colloq.)" , "Salmon" .)