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' is an insertion indicator.
'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)" .)