Timeless work about halfway into flat... (9)
I believe the answer is:
evergreen
'timeless' is the definition.
(I've seen this before)
'work about halfway into flat' is the wordplay.
'work' becomes 'erg' (an erg is a unit of work done).
'about halfway' becomes 're' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'into' means one lot of letters goes inside another.
'flat' becomes 'even' (level).
'erg'+'re'='ergre'
'ergre' going inside 'even' is 'EVERGREEN'.
Can you help me to learn more?
(Other definitions for evergreen that I've seen before include "song that's been popular for a long time" , "Having year-round foliage" , "Standard - type of shrub" , "Laurel, for example" , "Pine, for example" .)