Guy returning if accepted by centre left and groovy extremists (6)
I believe the answer is:
effigy
'guy' is the definition.
(as in an effigy of Guy Fawkes)
'returning if accepted by centre left and groovy extremists' is the wordplay.
'returning' says the letters should be written backwards.
'accepted by' is an insertion indicator (inserted letters are accepted).
'centre' indicates the central letters.
'and' means one lot of letters go next to another.
'extremists' says to hollow out the word (remove centre letters) (letters on the extreme ends of the word).
The central letters of 'left' are 'ef'.
'groovy' with its middle taken out is 'gy'.
'if' back-to-front is 'fi'.
'ef'+'gy'='efgy'
'fi' going into 'efgy' is 'EFFIGY'.
(Other definitions for effigy that I've seen before include "Likeness of a person in sculpture" , "Crude dummy of person" , "A guy, for instance" , "Image of a person (Guy Fawkes on 5 November?)" , "Graven image" .)