Smile, sir, about a silly MEP (6)
I believe the answer is:
simper
'smile' is the definition.
(I know that simper is a more specific form of the action smile)
'sir about a silly mep' is the wordplay.
'about' indicates putting letters inside.
'a silly' indicates an anagram.
'mep' anagrammed gives 'mpe'.
'sir' enclosing 'mpe' is 'SIMPER'.
(Other definitions for simper that I've seen before include "Silly self-conscious smile" , "Smile ingratiatingly" , "Smile in a coy manner" , "Primes to laugh affectedly" , "Smile self-consciously" .)