Forthright and mostly sad, not heartless (5)
I believe the answer is:
blunt
'forthright' is the definition.
(I've seen this before)
'mostly sad not heartless' is the wordplay.
'mostly' means to remove the last letter.
'sad' becomes 'blue' (similar in meaning).
'not heartless' becomes 'nt' (I am not sure about this - if you are sure you should believe this answer much more).
'blue' with its final letter taken off is 'blu'.
'blu'+'nt'='BLUNT'
'and' is the link.
(Other definitions for blunt that I've seen before include "To the point; without point" , "6 [JOHNSON]" , "Forthright - Soviet spy" , "Dull - outspoken - spy" , "Forthright in manner" .)