Forgive Conservative breaking old custom (6)
I believe the answer is:
excuse
'forgive' is the definition.
(I know that excuse is a more specific form of the action forgive)
'conservative breaking old custom' is the wordplay.
'conservative' becomes 'C' (UK political abbreviation).
'breaking' means one lot of letters goes inside another.
'old' becomes 'ex' (prefix meaning former or onetime).
'custom' becomes 'use' (use is a kind of custom).
'ex'+'use'='exuse'
'c' inserted into 'exuse' is 'EXCUSE'.
(Other definitions for excuse that I've seen before include "Pardon or forgive" , "Make allowances for; forgive" , "Allow to be released from a duty" , "Grant exemption or release to" , "Supposed reason for failure" .)