Regret picking up half the letters for nuclear organisation (7)
I believe the answer is:
euratom
'nuclear organisation' is the definition.
The definition suggests a singular noun which matches the answer.
'regret picking up half the letters' is the wordplay.
'regret' becomes 'rue' (to rue something is to regret it).
'picking up' is a reversal indicator (in a down clue, letters go up).
'half the letters' becomes 'atom' (A to M, the first 13 letters of the alphabet).
'rue' written backwards gives 'eur'.
'eur'+'atom'='EURATOM'
'for' is the link.
Can you help me to learn more?
(Other definitions for euratom that I've seen before include "nuclear researchers" , "Continental nuclear community" , "community association" .)