Dream when tart embraces redhead (6)
I believe the answer is:
aspire
'dream' is the definition.
(I've seen this before)
'when tart embraces redhead' is the wordplay.
'when' becomes 'as'.
'tart' becomes 'pie' (I've seen this before).
'embraces' indicates putting letters inside.
'redhead' becomes 'r' (the head/first letter of 'red').
'pie' enclosing 'r' is 'pire'.
'as'+'pire'='ASPIRE'
(Other definitions for aspire that I've seen before include "Hanker" , "Persia (anag.)" , "Aim at, work towards" , "Aim, shoot for" , "Aim at high things" .)