Nasty creature appears in this half-light (5)
I believe the answer is:
torch
'light' is the definition.
(type of light)
'nasty creature appears in this half' is the wordplay.
'nasty creature' becomes 'orc' (type of creature in The Lord of the Rings).
'appears in' indicates putting letters inside.
'half' suggests halving the letters.
'this' with half the letters taken is 'th'.
'orc' going within 'th' is 'TORCH'.
(Other definitions for torch that I've seen before include "Portable lamp; set alight" , "Portable light; burn" , "Link" , "Small hand-held battery lamp" , "Set fire to - hand-held light" .)