A weapon concealed in a deep eerie place (4)
I believe the answer is:
epee
'a weapon' is the definition.
(I know that epee is a type of fencing sword)
'concealed in a deep eerie place' is the wordplay.
'concealed in' indicates the answer is hidden within the clue.
'EPEE' is hidden within 'deep eerie place'.
(Other definitions for epee that I've seen before include "It may offer riposte" , "Type of sword" , "sports equipment" , "Olympic event" , "Rapier" .)