Palm marks of 51 cats (9)
I believe the answer is:
lifelines
'palm marks' is the definition.
(I've seen this in another clue)
'51 cats' is the wordplay.
'51' becomes 'LI' (roman numeral).
'cats' becomes 'felines' (I've seen this in another clue).
'li'+'felines'='LIFELINES'
'of' acts as a link.
(Other definitions for lifelines that I've seen before include "Means to rescue" , "May 23 [COME TO THE RESCUE], these" .)