The first half of the alphabet at first is centrepiece of elemental building blocks (6)
I believe the answer is:
atomic
'the first half of the alphabet at first is centrepiece of elemental building blocks' is the definition.
I can't tell whether this defines the answer.
'the first half of the alphabet at first is centrepiece' is the wordplay.
'the first half of the alphabet' becomes 'atom' (I've seen this in another clue).
'at first' says to take the initial letters.
The first letters of 'is centrepiece' is 'ic'.
'atom'+'ic'='ATOMIC'
Can you help me to learn more?
(Other definitions for atomic that I've seen before include "nuclear issues?" , "Referring to a tiny particle" , "Basically simple" , "Relating to nuclear energy" , "Of very small particles" .)