Pitcher's third mound is a hit (5)
I believe the answer is:
thump
'a hit' is the definition.
(I know that thump is a more specific form of the action hit)
'pitcher's third mound' is the wordplay.
'pitcher's third' becomes 't' (mrd letter of 'pitcher').
'mound' becomes 'hump' (I've seen this in another clue).
't'+'hump'='THUMP'
'is' acts as a link.
(Other definitions for thump that I've seen before include "Punch" , "Whack" , "Hit hard" , "Heavy blow with the hand" , "clobber" .)