Liar Boris in NHS U-turns? Terrible time (5,10)
I believe the answer is:
annus horribilis
'terrible time' is the definition.
(Latin phrase for a horrible year)
'liar boris in nhs u-turns?' is the wordplay.
'turns?' indicates anagramming the letters (I've seen 'turns' mean this).
'liar'+'boris'+'in'+'nhs'+'u'='liarborisinnhsu'
'liarborisinnhsu' with letters rearranged gives 'ANNUS HORRIBILIS'.
(Other definitions for annus horribilis that I've seen before include "It's really not a good time" , "Bad time" , "period when things are going badly" .)