This is a source of strikes (4)
I believe the answer is:
hits
'strikes' is the definition.
(I know this)
'this is a source' is the wordplay.
'is a source' indicates anagramming the letters.
'this' with letters rearranged gives 'HITS'.
'of' is the link.
(Other definitions for hits that I've seen before include "Pop successes" , "The sort of strikes" , "In the 60s these were 'groovy'" , "Top pops" , "that please the public?" .)