Philosopher’s stratagem involving singular lines (7)
I believe the answer is:
russell
'philosopher's' is the definition.
(Bertrand Russell)
'stratagem involving singular lines' is the wordplay.
'stratagem' becomes 'ruse' (a ruse is a stratagem or ploy).
'involving' means one lot of letters goes inside another.
'singular' becomes 's' (abbreviation in grammar).
'lines' becomes 'll' (an abbreviation for line written twice).
'ruse' going around 's' is 'russe'.
'russe'+'ll'='RUSSELL'
(Other definitions for russell that I've seen before include "Victorian PM" , "2 [PHILOSOPHER]" , "Jack -, type of terrier" , "See 2" , "Ken -, Eng. film director (Women in Love)" .)