Remember I'm sincere when upset (9)
I believe the answer is:
reminisce
'remember' is the definition.
(reminiscing is a kind of remembering)
'i'm sincere when upset' is the wordplay.
'when upset' indicates anagramming the letters.
'im'+'sincere'='imsincere'
'imsincere' anagrammed gives 'REMINISCE'.
(Other definitions for reminisce that I've seen before include "Talk about old times - I'm sincere (anag)" , "nostalgic" , "Give account of past" , "Think of past events" , "Talk like an oldie" .)