File about longest sentence (4)
I believe the answer is:
life
'longest sentence' is the definition.
(I've seen this before)
'file about' is the wordplay.
'about' indicates an anagram.
'file' with letters rearranged gives 'LIFE'.
(Other definitions for life that I've seen before include ""But an empty dream" (Longfellow) - "As tedious as a twice-told tale" (Shakespeare)" , "The longest sentence" , "Baffin oiler (anag) - 7 film" , "Animation" , "Existence - biography" .)