Smile ingratiatingly (6)
I believe the answer is:
'smile ingratiatingly' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for simper that I've seen before include "Primes to laugh affectedly" , "Primes (anag.)" , "Smile self-consciously" , "Smile in an affected manner" , "Smirk" .)