One at half mast is quick to sink! (5)
I believe the answer is:
stone
'quick to sink' is the definition.
I can't judge whether this defines the answer.
'one at half mast' is the wordplay.
'at' means one lot of letters go next to another (I've seen this in other clues).
'half' means to take half the letters.
'mast' with half the letters taken is 'st'.
'one' after 'st' is 'STONE'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for stone that I've seen before include "Boulder" , "Rock; fruit seed" , "Beryl, ruby" , "See 19" , "See 5" .)