Had better be unusually tough (5)
I believe the answer is:
ought
'had better' is the definition.
(synonyms)
'unusually tough' is the wordplay.
'unusually' indicates an anagram.
'tough' with letters rearranged gives 'OUGHT'.
'be' acts as a link.
(Other definitions for ought that I've seen before include "Strange thing" , "Should, had, better" , "Tough (anag)" , "Have/has a duty (to do)" , "Has a duty (to do something)" .)