Little man invests million that's mine! (4)
I believe the answer is:
bomb
'mine' is the definition.
(I've seen this before)
'little man invests million' is the wordplay.
'little man' becomes 'bob' (short for Robert. I am not sure about the 'little' bit.).
'invests' means one lot of letters goes inside another (inserted letters are invested).
'million' becomes 'm' (abbreviation).
'bob' placed around 'm' is 'BOMB'.
'that's' is the link.
Can you help me to learn more?
(Other definitions for bomb that I've seen before include "charge" , "(Of a theatre production) fail badly" , "Incendiary device" , "Load of money (slang)" , "See 18" .)