Drab and dull old woman depresses you, first of all (5)
I believe the answer is:
Here is my best explanation:
'drab' is the definition.
(I've seen this before)
'dull old woman depresses you first of all' is the subsidiary indication.
'first of all' suggests taking the first letters.
The first letters of 'dull old woman depresses you' is 'dowdy'.
'and' is the link.
(Other definitions for dowdy that I've seen before include "Frumpish", "Lacking chic", "Drab", "Unsmart" and "Drably dressed".)