Agree to attempt short back and sides for narcissist (6)
I believe the answer is:
assent
'agree to' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
(Other definitions for assent that I've seen before include "Permit or approve" , "Okay" , "Acquiescence" , "Accede, acquiesce" , "Agreement or concurrence" .)