Philosopher's almost impossible to get into (5)
I believe the answer is:
locke
'impossible to get into' is the definition.
I can't judge whether this defines the answer.
'philosopher's almost' is the wordplay.
'philosopher's' becomes 'lockes' (John Locke).
'almost' means to remove the last letter (most of the word but not all of it).
'lockes' with its final letter taken away is 'LOCKE'.
Can you help me to learn more?
(Other definitions for locke that I've seen before include "John -, seventeenth to eighteenth century English philosopher" , "English empiricist philosopher, d. 1704" , "A philosopher; sounds like strand of hair" .)