Remember, I’m sincere in a fashion (9)
I believe the answer is:
reminisce
'remember' is the definition.
(I know that reminisce is a more specific form of the action remember)
'i'm sincere in a fashion' is the wordplay.
'in a fashion' indicates an anagram (letters fashioned differently).
'im'+'sincere'='imsincere'
'imsincere' with letters rearranged gives 'REMINISCE'.
(Other definitions for reminisce that I've seen before include "Indulge in recollection" , "Recollect past events" , "Give account of past" , "talk about the old days" , "Remember past" .)