Essential getting elected, then great time for a U-turn! (8)
I believe the answer is:
integral
'essential' is the definition.
(thesaurus)
'elected then great time for a u-turn' is the wordplay.
'elected' becomes 'in'.
'then' is a charade indicator (letters next to each other).
'great' becomes 'large' ('large' can be a synonym of 'great'**).
'time for' becomes 't' (I've seen this in other clues).
'a u-turn' says the letters should be written backwards.
'large'+'t'='larget'
'larget' backwards is 'tegral'.
'in'+'tegral'='INTEGRAL'
'getting' acts as a link.
(Other definitions for integral that I've seen before include "Basic" , "Entire, whole" , "Mathematical quantity" , "Forming part of the whole" , "Fundamental; function" .)