Playwright’s maxim about hearts (4)
I believe the answer is:
shaw
'playwright's' is the definition.
(George Bernard Shaw)
'maxim about hearts' is the wordplay.
'maxim' becomes 'saw' (I've seen this in another clue).
'about' means one lot of letters goes inside another.
'hearts' becomes 'h' (abbreviation used in card games e.g. bridge).
'saw' enclosing 'h' is 'SHAW'.
(Other definitions for shaw that I've seen before include "Man and Superman playwright" , "Irish playwright, d. 1950" , "Irish author" , "making an appearance in Craiglockhart?" , "G. B. ---; T. E. ---" .)