Serious young Arab maybe secures room (6)
I believe the answer is:
formal
'serious' is the definition.
(thesaurus)
'young arab maybe secures room' is the wordplay.
I cannot quite see how this works, but
'room' could be 'rm' (abbreviation for room) and 'rm' is found in the answer.
'young' could be 'foal' (foal is a kind of young) and 'foal' is located in the remaining letters.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for formal that I've seen before include "Like court dress, perhaps" , "Conforming to accepted customs, say dress" , "Ceremonial; ordered" , "Done in accordance with etiquette" , "conventionally correct" .)