Dynamic pieces of trap start to open up (2,3)
I believe the answer is:
op art
'dynamic pieces of' is the definition.
The definition suggests a singular noun which matches the answer.
'trap start to open up' is the wordplay.
'start to' indicates taking the first letters.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
The first letter of 'open' is 'o'.
'trap'+'o'='trapo'
'trapo' in reverse letter order is 'OP-ART'.
Can you help me to learn more?
(Other definitions for op art that I've seen before include "Illusory genre" , "sort of painting" , "It may give the impression of movement" , "Created illusions" , "Style of optical illusionism popular in the 1960s" .)