It turned dark, turning fearful (5)
I believe the answer is:
timid
'fearful' is the definition.
(associated in meaning)
'it turned dark turning' is the wordplay.
'turned' shows that the letters should be reversed in order.
'dark' becomes 'dim' (similar in meaning).
'turning' shows that the letters should be reversed in order.
'it' back-to-front is 'ti'.
'dim' backwards is 'mid'.
'ti'+'mid'='TIMID'
(Other definitions for timid that I've seen before include "Lacking in courage" , "Gutless" , "Not assertive, retiring" , "Easily alarmed, shy" , "Lacking self-confidence" .)