He conceivably needs flag raised (3)
I believe the answer is:
gas
'he conceivably' is the definition.
The definition suggests a singular noun but the answer is a plural noun.
'flag raised' is the wordplay.
'flag' becomes 'sag' (synonyms).
'raised' shows that the letters should be reversed in order (going up in a down clue).
'sag' in reverse letter order is 'GAS'.
'needs' acts as a link.
Can you help me to learn more?
(Other definitions for gas that I've seen before include "Something that is neither solid not liquid" , "Nitrogen, say" , "Hydrogen or methane, say" , "Fuel; idle talk" , "eg CO2" .)