Positive American's sincerity is not all there (3-2)
I believe the answer is:
can-do
'positive' is the definition.
(I've seen this before)
'sincerity is not all there' is the wordplay.
'sincerity' becomes 'candor' (I've seen this in another clue).
'is not all there' means to remove the last letter.
'candor' with its last letter taken away is 'CAN-DO'.
'american's' is the link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for can-do that I've seen before include "Positive-thinking" , "Showing willingness to get the job done" , "Positive" .)