Blade going round front and back of knife (4)
I believe the answer is:
vane
'blade going round' is the definition.
'front and back of knife' is the wordplay.
'front' becomes 'van' (I've seen this before).
'and' is a charade indicator (letters next to each other).
'back of' indicates one should take the final letters.
The last letter of 'knife' is 'e'.
'van'+'e'='VANE'
(Other definitions for vane that I've seen before include "Blade of a propeller" , "Indicator of wind direction" , "The weather-cock sounds conceited" , "Windmill sail in 30 across" , "Wind pointer" .)