Open all the time after resistance ended at front (7)
I believe the answer is:
sincere
'open' is the definition.
(I've seen this before)
'all the time after resistance ended at front' is the wordplay.
'all the time after' becomes 'since'.
'at front' suggests taking the first letters.
The first letters of 'resistance ended' is 're'.
'since'+'re'='SINCERE'
(Other definitions for sincere that I've seen before include "natural?" , "Honest, heartfelt" , "Not hypocritical" , "straightforward" , "Unaffected" .)