Title used to identify data line changed in importance (8)
I believe the answer is:
filename
'changed in importance' is the definition.
The definition suggests a singular noun which matches the answer.
'title used to identify data line' is the wordplay.
I cannot really understand how this works, but
'title' could be 'name' (I've seen this before) and 'name' is found within the answer.
'line' could be 'file' (file is a kind of line) and 'file' is found in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Another definition for filename that I've seen is " Computer record identifier".)