Pessimist occasionally crying is melancholic at heart (5)
I believe the answer is:
cynic
'pessimist' is the definition.
(thesaurus)
'occasionally crying is melancholic at heart' is the wordplay.
'occasionally' indicates alternate letters.
'at heart' means to look at the middle letters.
The central letter of 'melancholic' is 'c'.
'crying'+'is'='cryingis'
The alternate letters of 'cryingis' are 'cyni'.
'cyni'+'c'='CYNIC'
(Other definitions for cynic that I've seen before include "One believing all are motivated by self-interest" , "One with a low view of human nature" , "Someone who believes the worst of others" , "Ancient philosopher" , "Doubter of others' motives" .)