Philosopher’s informal apology including staple answer (7)
I believe the answer is:
spinoza
'philosopher's' is the definition.
(I've seen this before)
'informal apology including staple answer' is the wordplay.
'informal apology' becomes 'soz' (slang version of 'sorry').
'including' means one lot of letters goes inside another.
'staple' becomes 'pin'.
'answer' becomes 'a' (abbreviation eg in Q and A).
'soz' enclosing 'pin' is 'spinoz'.
'spinoz'+'a'='SPINOZA'
(Other definitions for spinoza that I've seen before include "philosophical sort" , "2 [PHILOSOPHER]" , "C17 Dutch philosopher" , "Dutch 17th-century philosopher" .)