Seniors note park runs are ruthless (4,2,9)
I believe the answer is:
take no prisoners
'ruthless' is the definition.
(I've seen this in another clue)
'seniors note park runs' is the wordplay.
'runs' indicates an anagram.
'seniors'+'note'+'park'='seniorsnotepark'
'seniorsnotepark' is an anagram of 'TAKE NO PRISONERS'.
'are' acts as a link.
(Other definitions for take no prisoners that I've seen before include "Ruthlessly pursue aim" , "reduce numbers held in gaol?" .)