Take courage, try size down from twelve (7)
I believe the answer is:
hearten
'take courage' is the definition.
'try size down from twelve' is the wordplay.
'try' becomes 'hear' (as in to try someone in court or hear their case**).
'size down from twelve' becomes 'ten' (UK clothing sizes).
'hear'+'ten'='HEARTEN'
(Other definitions for hearten that I've seen before include "giving cheer" , "Encourage or cheer in cardiac manner" , "Make cheerful" , "Comfort" , "Make (someone) more confident" .)