Tough to hide one old cut (6)
I believe the answer is:
hairdo
'cut' is the definition.
(I've seen this before)
'tough to hide one old' is the wordplay.
'tough' becomes 'hard' (associated in meaning).
'to hide' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'hard' going around 'i' is 'haird'.
'haird'+'o'='HAIRDO'
(Other definitions for hairdo that I've seen before include "Possibly a DA" , "Cut and set" , "Style for one's crowning glory" , "The process of styling hirsute head" , "Possibly cut" .)