Demure English principal (5)
I believe the answer is:
prime
'principal' is the definition.
(thesaurus)
'demure english' is the wordplay.
'demure' becomes 'prim' (I've seen this before).
'english' becomes 'e' (abbreviation).
'prim'+'e'='PRIME'
(Other definitions for prime that I've seen before include "Main; make ready" , "Maybe eleven" , "Prepare to fire - first" , "Of first quality" , "Number with no factor but itself and 1" .)