Prime Minister to declare U-turn and modernise (6)
I believe the answer is:
revamp
'modernise' is the definition.
(I've seen this in another clue)
'prime minister to declare u-turn' is the wordplay.
'prime minister' becomes 'PM' (abbreviation).
'to declare' becomes 'aver' (averring is a kind of declaring).
'u-turn' says the letters should be written backwards.
'pm'+'aver'='pmaver'
'pmaver' backwards is 'REVAMP'.
'and' acts as a link.
(Other definitions for revamp that I've seen before include "get in better shape" , "face-lift" , "Refurbish" , "overhaul" , "Patch up or renovate" .)