Move aside, losing singular notion (4)
I believe the answer is:
idea
'notion' is the definition.
(both can mean a thought)
'move aside losing singular' is the wordplay.
'move' indicates anagramming the letters (I've seen 'moving' mean this).
'losing' is a deletion indicator.
'singular' becomes 's' (abbreviation sometimes used in dictionaries).
'aside' with 's' taken out is 'aide'.
'aide' is an anagram of 'IDEA'.
(Other definitions for idea that I've seen before include "Sudden inspiration?" , "Mental impression" , "intellectual product" , "a wheeze" , "Notion, concept" .)