Show welcoming resistance with Boris's last U-turn (8)
I believe the answer is:
reversal
'u-turn' is the definition.
(I've seen this before)
'show welcoming resistance with boris's last' is the wordplay.
'show' becomes 'reveal' (revealing is a kind of showing**).
'welcoming' indicates putting letters inside (in sense of 'inviting in').
'resistance' becomes 'r' (abbreviation).
'with' is a charade indicator (letters next to each other).
'last' indicates one should take the final letters.
The last letter of 'boriss' is 's'.
'r'+'s'='rs'
'reveal' going around 'rs' is 'REVERSAL'.
(Other definitions for reversal that I've seen before include "A turning back" , "Turnabout" , "A turning backwards" , "complete change" , "Change of fortune" .)