Bug's beginning to crawl into cracked light (6)
I believe the answer is:
glitch
'bug's' is the definition.
('glitch' can be a synonym of 'bug')
'beginning to crawl into cracked light' is the wordplay.
'beginning to' suggests taking the first letters.
'into' indicates putting letters inside.
'cracked' is an anagram indicator.
The first letter of 'crawl' is 'c'.
'light' anagrammed gives 'glith'.
'c' placed into 'glith' is 'GLITCH'.
(Other definitions for glitch that I've seen before include "Small fault preventing proper working" , "Irregular computer failure" , "Minor mishap" , "Temporary fault" , "Brief failure" .)