Dowdiness primarily missing in tailored threads -- suit! (6)
I believe the answer is:
'suit' is the definition.
(hearts is a playing card suit)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for hearts that I've seen before include "Card suit and essential organs" , "Playing-card suit" , "Scots club nickname; suit" , "Cores" , "Major suit at cards" .)