One investing in Deutschmark? Not bright (3)
I believe the answer is:
dim
'not bright' is the definition.
(I've seen this in another clue)
'one investing in deutschmark?' is the wordplay.
'one' becomes 'i' (Roman numeral).
'investing in' means one lot of letters goes inside another.
'deutschmark?' becomes 'dm' (I can't justify this - if you can you should give a lot more credence to this answer).
'i' placed within 'dm' is 'DIM'.
Can you help me to learn more?
(Other definitions for dim that I've seen before include "& 26 Chinese food selection" , "Indistinct, dark" , "Lacking clarity or distinctness" , "Not brilliant" , "Not bright, poorly lit" .)