Hard going by single file (4)
I believe the answer is:
hone
'file' is the definition.
(thesaurus)
'hard going by single' is the wordplay.
'hard' becomes 'h'.
'going by' is a charade indicator (letters next to each other).
'single' becomes 'one' (I've seen this before).
'h'+'one'='HONE'
(Other definitions for hone that I've seen before include "Strop" , "Polish" , "Sharpen tool on stone" , "Sharpen (a blade)" , "Smooth stone" .)