Defence of Post Office record one’s put in two answers (8)
I believe the answer is:
apologia
'defence' is the definition.
(apologia is a kind of defence)
'post office record one's put in two answers' is the wordplay.
'post office' becomes 'po' (abbreviation).
'record' becomes 'log' (log is a kind of record).
'one' becomes 'i' (Roman numeral).
'put in' is an insertion indicator.
'two answers' becomes 'AA' (an abbreviation for answer, twice).
'po'+'log'+'i'='pologi'
'pologi' placed within 'aa' is 'APOLOGIA'.
'of' acts as a link.
(Other definitions for apologia that I've seen before include "Formal defence of one's actions" , "Written defence of one's conduct" , "Formal defence of one's conduct" , "form of defence" , "Formal written defence of one's opinions/conduct" .)