Giant-killer drunkard keeping Heather quiet? (9)
I believe the answer is:
slingshot
'giant-killer' is the definition.
(how David killed Goliath in the Bible)
'drunkard keeping heather quiet?' is the wordplay.
'drunkard' becomes 'sot' (synonyms).
'keeping' means one lot of letters goes inside another.
'heather' becomes 'ling' (synonyms).
'quiet?' becomes 'sh' (both can be said to tell someone to be quiet).
'ling'+'sh'='lingsh'
'sot' going around 'lingsh' is 'SLINGSHOT'.
(Other definitions for slingshot that I've seen before include "fatal to Goliath" , "that can help you get a stone off" , "Sing Holst? (Anag)" , "Catapult -- thin gloss (anag)" , "Weapon American kid may produce" .)