Good tonsure is almost a disaster! (5,5)
I believe the answer is:
close shave
'almost a disaster' is the definition.
(I've seen this before)
'good tonsure' is the wordplay.
'good' becomes 'close' (similar in meaning).
'tonsure' becomes 'shave' (I've seen this before).
'close'+'shave'='CLOSE SHAVE'
'is' is the link.
(Other definitions for close shave that I've seen before include "nasty moment" , "Almost a disaster" , "Danger narrowly averted" , "result being not so hairy" , "resulting in barefaced cheek?" .)