Be careful to make a mark in shares (6)
I believe the answer is:
scrimp
'be careful to' is the definition.
I can't tell whether this defines the answer.
'a mark in shares' is the wordplay.
'a mark' becomes 'm'.
'in' means one lot of letters goes inside another.
'shares' becomes 'scrip' (I have seen 'extra shares' mean 'scrip' so perhaps 'shares' could also mean 'scrip').
'm' placed inside 'scrip' is 'SCRIMP'.
'make' is the link.
Can you help me to learn more?
(Other definitions for scrimp that I've seen before include "Be economical, mean, with" , "Pinch and save" , "Be frugal and sparing" , "Make too small, stint" , "To pinch pennies" .)