Prepared to be demure with a boy (6)
I believe the answer is:
primed
'prepared' is the definition.
(priming is a kind of preparing)
'demure with a boy' is the wordplay.
'demure' becomes 'prim' (I've seen this before).
'with' is a charade indicator (letters next to each other).
'a boy' becomes 'ed' (short for edward).
'prim'+'ed'='PRIMED'
'to be' acts as a link.
(Other definitions for primed that I've seen before include "Briefed - ready" , "charged" , "Prepared something for use or action" , "Made ready (to fire)" , "Filled in" .)