They fear flickering light (8)
I believe the answer is:
feathery
'light' is the definition.
(similar in meaning)
'they fear flickering' is the wordplay.
'flickering' is an anagram indicator (flickering is a type of quick movement).
'they'+'fear'='theyfear'
'theyfear' is an anagram of 'FEATHERY'.
(Other definitions for feathery that I've seen before include "being described as down" , "Fluffy" , "like birds" .)