The sharp side in the back of the peg desk (4)
I believe the answer is:
edge
'the sharp side' is the definition.
(I've seen this in another clue)
'back of the peg desk' is the wordplay.
'back' says the letters should be written backwards.
'of' indicates the answer is hidden within the clue.
'egde' can be found hidden inside 'peg desk'.
'egde' reversed gives 'EDGE'.
'in the' is the link.
(Other definitions for edge that I've seen before include "Sidle, lip" , "Brim" , "Surface boundary" , "small advantage" , "Move little by little to the border" .)