See there's a knack in standing upright (8)
I believe the answer is:
straight
'upright' is the definition.
(synonyms)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for straight that I've seen before include "In a direct line" , "Serious" , "Without deviation" , "(Of drink) undiluted" , "Without deviations or curves" .)