Best place to make contact with darling Mark (5,4)
I believe the answer is:
sweet spot
'best place to make contact' is the definition.
(I've seen this before)
'darling mark' is the wordplay.
'darling' becomes 'sweet' (I've seen this before).
'mark' becomes 'spot' (spotting is a kind of marking).
'sweet'+'spot'='SWEET SPOT'
'with' acts as a link.
(Other definitions for sweet spot that I've seen before include "Best place to make contact" , "Best area of a tennis or squash racket" , "Best area on, eg, tennis racket" , "Most effective section of racket" , "Best point of racket or bat when hitting the ball" .)