A few invest nothing in a common kind of enterprise (4)
I believe the answer is:
some
'a few' is the definition.
(I've seen this before)
'nothing in a common kind of enterprise' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'in' is an insertion indicator.
'a common kind of enterprise' becomes 'sme' (short for Small and Medium-sized Enterprises ).
'o' put within 'sme' is 'SOME'.
'invest' is the link.
I am not very happy about this link. It may belong to another bit of the clue.
(Other definitions for some that I've seen before include "Undefined number" , "A little; not a little" , "certain" , "Hard work" , "Part" .)