Tolerate hairstyle — one time! (6)
I believe the answer is:
permit
'tolerate' is the definition.
(I know that tolerate can be written as permit)
'hairstyle one time' is the wordplay.
'hairstyle' becomes 'perm' (perm is a hairstyle).
'one' becomes 'I' (Roman numeral).
'time' becomes 't' (abbreviation).
'perm'+'i'+'t'='PERMIT'
(Other definitions for permit that I've seen before include "Let - pass" , "Allow - visa" , "Warrant" , "Allow, authorise" , "Licence -- license" .)