One expecting the right to wear second medal (6-2-2)
I believe the answer is:
mother-to-be
'one expecting' is the definition.
'the right to wear second medal' is the subsidiary indication.
'right' becomes 'rt'.
'to wear' means one lot of letters goes inside another.
'second' becomes 'mo' (both are short periods of time).
'medal' becomes 'obe'.
'the'+'rt'='thert'
'mo'+'obe'='moobe'
'thert' inserted into 'moobe' is 'mothertobe'.
(Other definitions for mother-to-be that I've seen before include "She's expecting" , "Expecting woman?" , "One expecting a baby" .)